Korean War

In South Korea, the war is usually referred to as the "625 War" (6·25 전쟁; 六二五戰爭; Yugio jeonjaeng), the "625 Upheaval" (6·25 동란; 六二五動亂; Yugio dongnan), or simply "625", reflecting the date of its commencement on 25 June.

In North Korea, the war is officially referred to as the "Fatherland Liberation War" (조국해방전쟁; Choguk haebang chŏnjaeng) or commonly the "Chosŏn [Korean] War" (조선전쟁; Chosŏn chŏnjaeng).

In the United States, the war was initially described by President Harry S. Truman as a "police action" as the US never formally declared war and its military actions in Korea were conducted under the auspices of the UN flag. It has sometimes been referred to in the English-speaking world as "The Forgotten War" or "The Unknown War" because of the lack of public attention it received relative to World War II and the Vietnam War.

In China, the segment of the war after the intervention of the People's Volunteer Army is commonly and officially known as the "War to Resist America and Assist Korea" (Chinese: 抗美援朝战争; pinyin: Kàngměi Yuáncháo Zhànzhēng), while the segment preceding that is officially called the "Korean Civil War" (simplified Chinese: 朝鲜内战; traditional Chinese: 朝鮮內戰; pinyin: Cháoxiǎn Nèizhàn). The term "Chosŏn War" (simplified Chinese: 朝鲜战争; traditional Chinese: 朝鮮戰爭; pinyin: Cháoxiǎn Zhànzhēng) is sometimes used unofficially. The term "Hán (Korean) War" (Chinese: 韓戰; pinyin: Hán Zhàn) is most used in Taiwan (Republic of China), Hong Kong and Macau.

Korea was a colony ruled by the Empire of Japan from 1910 to 1945. The Empire of Japan diminished China's influence over Korea in the First Sino-Japanese War (1894–95). A decade later, after defeating Imperial Russia in the Russo-Japanese War, Japan made the Korean Empire its protectorate with the Eulsa Treaty in 1905, and then annexed it with the Japan–Korea Treaty of 1910.

Many Korean nationalists fled the country. The Provisional Government of the Republic of Korea was founded in 1919 in the Beiyang government. It failed to achieve international recognition, failed to unite the nationalist groups, and had a fractious relationship with its US-based founding president, Syngman Rhee.

The Chinese nationalist National Revolutionary Army and the communist People's Liberation Army (PLA) helped organize Korean refugees against the Japanese military, which had also occupied parts of China. The nationalist-backed Koreans, led by Yi Pom-Sok, fought in the Burma campaign (1941–1945). The communists, led by Kim Il Sung, among others, fought the Japanese in Korea and Manchuria.

At the Tehran Conference in 1943 and the Yalta Conference in February 1945, the Soviet Union promised to join the Allies in the Pacific War within three months of the victory in Europe. On 8 August 1945, the USSR declared war on Japan and invaded Manchuria. On 10 August, Soviet forces entered northern Korea and had secured most major cities in the north by 24 August, as Japanese resistance was light. Having fought Japan on Korean soil, the Soviet forces were well received by Koreans. Imperial Japanese rule of Korea officially ended when Japan surrendered to the Allies on 15 August 1945, following the atomic bombings of Hiroshima and Nagasaki.

At the Cairo Conference in 1943, China, the UK, and the US decided that "in due course, Korea shall become free and independent".

As World War II drew to a close, on 11 August 1945 two American military officials in Washington – US Colonels Dean Rusk and Charles H. Bonesteel III – chose an invisible line across the former Japanese colony, the 38th Parallel, for the division of Korea into two occupation zones, north and south, Soviet and American.

This was incorporated into US General Order No. 1, which responded to the Japanese surrender on 15 August. Joseph Stalin, however, maintained his wartime policy of cooperation, and on 16 August, the Red Army halted at the 38th parallel for three weeks to await the arrival of US forces. In explaining the choice of the 38th parallel, US Colonel Dean Rusk observed that, "Even though it was further north than could be realistically reached by US forces in the event of Soviet disagreement ... we felt it important to include the capital of Korea in the area of responsibility of American troops".

On 7 September 1945, General Douglas MacArthur issued Proclamation No. 1 to the people of Korea, announcing US military control over Korea south of the 38th parallel and establishing English as the official language during military control. On 8 September, US Lieutenant General John R. Hodge arrived in Incheon to accept the Japanese surrender south of the 38th parallel. Appointed as military governor, Hodge directly controlled South Korea as head of the US Army Military Government in Korea (USAMGIK 1945–48).

In December 1945, Korea was administered by the US–Soviet Union Joint Commission, as agreed at the Moscow Conference, to grant independence after a five-year trusteeship. Waiting five years for independence was unpopular among Koreans, and riots broke out. The Communist Party supported the trusteeship, while Kim Ku and Syngman Rhee led the anti-trusteeship movement against both the US Army Military Government in Korea and the Soviet military administration. To contain them, the USAMGIK banned strikes on 8 December and outlawed the PRK Revolutionary Government and People's Committees on 12 December. Following further civilian unrest, the USAMGIK declared martial law.

Citing the inability of the Joint Commission to make progress, the United Nations (UN) decided to hold an election under UN auspices to create an independent Korea, as stated in UN General Assembly Resolution 112. The Soviet authorities and Korean communists refused to participate in the election. The final attempt to establish a unified government was thwarted by North Korea's refusal. Due to concerns about division caused by an election without North Korea's participation, many South Korean politicians boycotted it. The 1948 South Korean general election was held in May. The resultant South Korean government promulgated a national political constitution on 17 July and elected Syngman Rhee as president on 20 July. The Republic of Korea (South Korea) was established on 15 August 1948.

In the Soviet-Korean Zone of Occupation, the Soviets agreed to the establishment of a communist government led by Kim Il Sung. The Democratic People's Republic of Korea (North Korea) was established on 10 July 1948. The 1948 North Korean parliamentary elections took place in August. The Soviet Union withdrew its forces in 1948 and the US in 1949.

With the end of the war with Japan, the Chinese Civil War resumed in earnest between the Communists and the Nationalist-led government. While the Communists were struggling for supremacy in Manchuria, they were supported by the North Korean government with matériel and manpower.

According to Chinese sources, the North Koreans donated 2,000 railway cars worth of supplies while thousands of Koreans served in the Chinese PLA during the war. North Korea also provided the Chinese Communists in Manchuria with a safe refuge for non-combatants and communications with the rest of China. As a token of gratitude, between 50,000 and 70,000 Korean veterans who served in the PLA were sent back along with their weapons, and they later played a significant role in the initial invasion of South Korea. China promised to support the North Koreans in the event of a war against South Korea.

By 1948, a North Korea-backed insurgency had broken out in the southern half of the peninsula. This was exacerbated by the undeclared border war between the Koreas, which saw division-level engagements and thousands of deaths on both sides. The ROK was almost entirely trained and focused on counterinsurgency, rather than conventional warfare. They were equipped and advised by a force of a few hundred American officers, who were successful in helping the ROKA to subdue guerrillas and hold its own against North Korean military (Korean People's Army, KPA) forces along the 38th parallel. Approximately 8,000 South Korean soldiers and police officers died in the insurgent war and border clashes.

The first socialist uprising occurred without direct North Korean participation, though the guerrillas still professed support for the northern government. Beginning in April 1948 on Jeju Island, the campaign saw arrests and repression by the South Korean government in the fight against the South Korean Labor Party, resulting in 30,000 violent deaths, among them 14,373 civilians, of whom 2,000 were killed by rebels and 12,000 by ROK security forces. The Yeosu–Suncheon rebellion overlapped with it, as several thousand army defectors waving red flags massacred right-leaning families. This resulted in another brutal suppression by the government and between 2,976 and 3,392 deaths. By May 1949, both uprisings had been crushed.

Insurgency reignited in the spring of 1949 when attacks by guerrillas in the mountainous regions (buttressed by army defectors and North Korean agents) increased. Insurgent activity peaked in late 1949 as the ROKA engaged so-called People's Guerrilla Units. Organized and armed by the North Korean government, and backed by 2,400 KPA commandos who had infiltrated through the border, these guerrillas launched an offensive in September aimed at undermining the South Korean government and preparing the country for the KPA's arrival in force. This offensive failed. However, the guerrillas were now entrenched in the Taebaek-san region of the North Gyeongsang Province and the border areas of the Gangwon Province.

While the insurgency was ongoing, the ROKA and KPA engaged in battalion-sized battles along the border, starting in May 1949. Border clashes between South and North continued on 4 August 1949, when thousands of North Korean troops attacked South Korean troops occupying territory north of the 38th parallel. The 2nd and 18th ROK Infantry Regiments repulsed attacks in Kuksa-bong, and KPA troops were "completely routed". Border incidents decreased by the start of 1950.

Meanwhile, counterinsurgencies in the South Korean interior intensified; persistent operations, paired with worsening weather, denied the guerrillas sanctuary and wore away their fighting strength. North Korea responded by sending more troops to link up with insurgents and build more partisan cadres; North Korean infiltrators had reached 3,000 soldiers in 12 units by the start of 1950, but all were destroyed or scattered by the ROKA.

On 1 October 1949, the ROKA launched a three-pronged assault on the insurgents in South Cholla and Taegu. By March 1950, the ROKA claimed 5,621 guerrillas killed or captured and 1,066 small arms seized. This operation crippled the insurgency. Soon after, North Korea made final attempts to keep the uprising active, sending battalion-sized units of infiltrators under the commands of Kim Sang-ho and Kim Moo-hyon. The first battalion was reduced to a single man over the course of engagements by the ROKA 8th Division. The second was annihilated by a two-battalion hammer-and-anvil maneuver by units of the ROKA 6th Division, resulting in a toll of 584 KPA guerrillas (480 killed, 104 captured) and 69 ROKA troops killed, plus 184 wounded. By the spring of 1950, guerrilla activity had mostly subsided; the border, too, was calm.

By 1949, South Korean and US military actions had reduced indigenous communist guerrillas in the South from 5,000 to 1,000. However, Kim Il Sung believed widespread uprisings had weakened the South Korean military and that a North Korean invasion would be welcomed by much of the South Korean population. Kim began seeking Stalin's support for an invasion in March 1949, traveling to Moscow to persuade him. Kim and second-in-command Pak Hon-yong tried to enlist Soviet ambassador Terentii Shtykov. Kim met with the ambassador on 12 August 1949; Shtykov said that an outright invasion was out of the question unless South Korea attacked first and that he only willing to consider a limited operation targeting the Ongjin peninsula. Mun Il visited Soviet chargé d'affaires Grigory Tunkin on 3 September 1949. Tunkin's report to the Soviet government was more neutral than previous Soviet diplomats, who were more skeptical.

Stalin initially did not think the time was right for a war in Korea. PLA forces were still embroiled in the Chinese Civil War, while US forces remained stationed in South Korea. After learning about Kim and Pak's plans, he told Tunkin to talk to Kim to produce an assessment of the South Korean and North Korean armed forces. After assessing both sides, Tunkin suggested against an invasion, to which Stalin agreed to and also banned the limited strike on the Ongjin peninsula. Kim continued his insistence on an invasion. On 17 January 1950, he told Soviet diplomats that South Korea needed to be liberated.

By 1950, Stalin was more open to suggestions and believed that the strategic situation had changed: PLA forces under Mao Zedong had secured final victory, US forces had withdrawn from Korea, and the Soviets had detonated their first nuclear bomb, breaking the US monopoly. As the US had not directly intervened to stop the communists in China, Stalin calculated they would be even less willing to fight in Korea, which had less strategic significance.

At 4:40 a.m. on 25 June 1950, the KPA crossed the 38th parallel behind artillery fire. The DPRK did not declare war before the invasion (called Operation Pokpung) and rushed to encircle and eventually capture Seoul, the capital of South Korea, from the ROK within a week. KPA forces swarmed South Korea and attacked all along the 38th parallel within an hour, and individual KPA units had advanced 3 to 5 kilometers into South Korean territory within the first three hours. The KPA had a combined arms force including tanks supported by heavy artillery supplied by the Soviet Union. The ROK had no tanks, anti-tank weapons, or heavy artillery. The South Koreans committed their forces in a piecemeal fashion, and these were routed in a few days.

North Korea justified its assault with the claim ROK troops attacked first and that the KPA were aiming to arrest and execute the "bandit traitor Syngman Rhee". Fighting began on the strategic Ongjin Peninsula in the west. There were initial South Korean claims that the 17th Regiment had counterattacked at Haeju; some scholars argue the claimed counterattack was instead the instigating attack, and therefore that the South Koreans may have fired first. However, the report that contained the Haeju claim contained errors and outright falsehoods.

On 27 June, Rhee evacuated Seoul with some of the government. On 28 June, the ROK blew up the Hangang Bridge across the Han River in an attempt to stop the KPA. The bridge was detonated while 4,000 refugees were crossing it and hundreds were killed. Destroying the bridge trapped many ROK units north of the river. In spite of such desperate measures, Seoul fell that same day. Some South Korean National Assemblymen remained in Seoul when it fell, and 48 subsequently pledged allegiance to the North.

Within days of the invasion, masses of ROK soldiers, of dubious loyalty to the Syngman Rhee regime, were retreating southwards or defecting en masse to the northern side, the KPA. On 28 June, Rhee ordered the massacre of suspected political opponents in his own country. In five days, the ROK, which had 95,000 troops on 25 June, was down to less than 22,000 troops. In early July, when US forces arrived, what was left of the ROK was placed under US operational command of the UN Command.

On 25 June 1950, the United Nations Security Council (UNSC) adopted Resolution 83 finding that the North Korean invasion of the Republic of Korea was a breach of the peace in violation of Chapter VII of the UN Charter. The Soviet Union, a veto-wielding power, had been boycotting the UN Security Council since January 1950 in protest of Taiwan's occupation of China's permanent seat. Thus, the Soviet Union's representative was not present at the meeting and was unable to vote against the resolution.

On 27 June, the Security Council adopted Resolution 83 recommending its member states provide military assistance to South Korea to restore international peace, resulting in a coalition led by the United States.

On 4 July, the Soviet Union's deputy foreign minister accused the US of starting an armed intervention on behalf of South Korea. The Soviet Union challenged the legitimacy of the war for several reasons. The ROK intelligence upon which UN Security Council Resolution 83 was based on came from US Intelligence; North Korea was not invited as a sitting temporary member of the UN, which violated Article 32 of the UN Charter; and the fighting was beyond the UN Charter's scope, because the initial North–South border fighting was classed as a civil war. Because the Soviet Union was boycotting the UN Security Council, some legal scholars posited that deciding upon this type of action required the unanimous vote of all five permanent members.

On 7 July, the Security Council adopted Resolution 84, which set up under the sponsorship of the British and French sides a Unified Command structure which was authorized to fly the blue flag of the UN but was not subject in any way to UN orders.

As soon as word of the attack was received, Acheson informed Truman that the North Koreans had invaded the Republic of Korea. Truman and Acheson discussed a US response to the invasion and agreed the US was obligated to act, comparing the North Korean invasion with Adolf Hitler's aggressions in the 1930s, and the mistake of appeasement must not be repeated. The loose notion of a "police action" was used by President Harry S. Truman as a legal pretext for ordering US troops to Korea in June 1950 without first requesting congressional authority. US industries were mobilized to supply materials, labor, capital, production facilities, and other services necessary to support the military objectives of the Korean War. Truman later explained he believed fighting the invasion was essential to the containment of communism as outlined in the National Security Council Report 68 (NSC 68):

Communism was acting in Korea, just as Hitler, Mussolini and the Japanese had ten, fifteen, and twenty years earlier. I felt certain that if South Korea was allowed to fall, Communist leaders would be emboldened to override nations closer to our own shores. If the Communists were permitted to force their way into the Republic of Korea without opposition from the free world, no small nation would have the courage to resist threats and aggression by stronger Communist neighbors.

In August 1950, Truman and Acheson obtained the consent of Congress to appropriate $12 billion for military action, equivalent to $161 billion in 2025. Because of the extensive defense cuts and emphasis on building a nuclear bomber force, none of the services were able to make a robust response with conventional military strength. General Omar Bradley, Chairman of the Joint Chiefs of Staff, was faced with deploying a force that was a shadow of its World War II counterpart.

Acting on Acheson's recommendation, Truman ordered MacArthur, the Supreme Commander for the Allied Powers in Japan, to transfer matériel to the South Korean military, while giving air cover to evacuation of US nationals. Truman disagreed with advisers who recommended unilateral bombing of the North Korean forces and ordered the US Seventh Fleet to protect Taiwan, whose government asked to fight in Korea. The US denied Taiwan's request for combat, lest it provoke retaliation from the PRC. Because the US had sent the Seventh Fleet to "neutralize" the Taiwan Strait, Chinese Premier Zhou Enlai criticized the UN and US initiatives as "armed aggression on Chinese territory". The US supported the Kuomintang in Burma in the hope these KMT forces would harass China from the southwest, thereby diverting Chinese resources from Korea.

"Korea was a critical presence in American policy at the dawn of the Cold War. The Truman administration identified its stake in Korea in the same 'fifteen weeks' in which the containment doctrine and the Marshall Plan were hammered out."

The Truman administration was worried a war in Korea could quickly escalate without American intervention. Diplomat John Foster Dulles stated: "To sit by while Korea is overrun by unprovoked armed attack would start a disastrous chain of events leading most probably to world war." While there was hesitance by some in the US government to get involved, considerations about Japan fed into the decision to engage on behalf of South Korea. After the fall of China to the communists, US experts saw Japan as the region's counterweight to the Soviet Union and China. While there was no US policy dealing with South Korea directly as a national interest, its proximity to Japan increased its importance. Said Kim: "The recognition that the security of Japan required a non-hostile Korea led directly to President Truman's decision to intervene ... The essential point ... is that the American response to the North Korean attack stemmed from considerations of US policy toward Japan."

Another consideration was the Soviet reaction if the US intervened. The Truman administration was fearful the Korean War was a diversionary assault that would escalate to a general war in Europe once the US committed in Korea. At the same time, "[t]here was no suggestion from anyone that the United Nations or the United States could back away from [the conflict]". Yugoslavia, a possible Soviet target because of the Tito–Stalin split, was vital to the defense of Italy and Greece, and the country was first on the list of the National Security Council's post-North Korea invasion list of "chief danger spots". Truman believed if aggression went unchecked, a chain reaction would start that would marginalize the UN and encourage communist aggression elsewhere. The UN Security Council approved the use of force to help the South Koreans. The Truman administration was uncertain whether the attack was a ploy by the Soviet Union, or just a test of US resolve. The decision to commit ground troops became viable when a communiqué was received on 27 June indicating the Soviet Union would not move against US forces in Korea. The Truman administration believed it could intervene in Korea without undermining its commitments elsewhere.

The Truman administration was unprepared for the invasion. Korea was not included in the strategic Asian Defense Perimeter outlined by US Secretary of State Dean Acheson. Military strategists were more concerned with the security of Europe against the Soviet Union than that of East Asia.

In postwar analysis of the unpreparedness of US forces deployed during the summer and fall of 1950, Army Major General Floyd L. Parks stated that, "Many who never lived to tell the tale had to fight the full range of ground warfare from offensive to delaying action, unit by unit, man by man ... [T]hat we were able to snatch victory from the jaws of defeat ... does not relieve us from the blame of having placed our own flesh and blood in such a predicament."

By 1950, US Secretary of Defense Louis A. Johnson had established a policy of faithfully following Truman's defense economization plans and aggressively attempted to implement it, even in the face of steadily increasing external threats. He consequently received much of the blame for the initial setbacks and widespread reports of ill-equipped and inadequately trained military forces in the war's early stages.

As an initial response to the invasion, Truman called for a naval blockade of North Korea and was shocked to learn that such a blockade could be imposed only "on paper" since the US Navy no longer had the warships with which to carry out his request. Army officials, desperate for weaponry, recovered Sherman tanks and other equipment from Pacific War battlefields and reconditioned them for shipment to Korea. Army ordnance officials at Fort Knox pulled down M26 Pershing tanks from display pedestals around Fort Knox in order to equip the third company of the Army's hastily formed 70th Tank Battalion. Without adequate numbers of tactical fighter-bomber aircraft, the US Air Force took F-51 (P-51) propeller-driven aircraft out of storage or from existing Air National Guard squadrons and rushed them into front-line service. A shortage of spare parts and qualified maintenance personnel resulted in improvised repairs and overhauls. A Navy helicopter pilot aboard an active duty warship recalled fixing damaged rotor blades with masking tape in the absence of spares.

US Army Reserve and Army National Guard infantry soldiers and new inductees (called to duty to fill out understrength infantry divisions) found themselves short of nearly everything needed to repel the North Korean forces: artillery, ammunition, heavy tanks, ground-support aircraft, even effective anti-tank weapons such as the M20 3.5-inch "Super Bazooka". Some Army combat units sent to Korea were supplied with worn-out, "red-lined" M1 rifles or carbines in immediate need of ordnance depot overhaul or repair.

The ground war saw armored offensives with the North's invasion and UN operations, and tunnel warfare by Chinese forces. The air war saw the first large jet aircraft battles, and an early use of US nuclear deterrence.

The initial assault by KPA forces was aided by the use of Soviet T-34-85 tanks. A KPA tank corps equipped with about 120 T-34s spearheaded the invasion. These faced an ROK that had few anti-tank weapons adequate to deal with the T-34s. Additional Soviet armor was added as the offensive progressed. The KPA tanks had a good deal of early successes against ROK infantry, Task Force Smith, and the US M24 Chaffee light tanks that they encountered. Interdiction by ground attack aircraft was the only means of slowing the advancing KPA armor. The tide turned in favor of the UN forces in August 1950 when the KPA suffered major tank losses during a series of battles in which the UN forces brought heavier equipment to bear, including American M4A3 Sherman and M26 medium tanks, alongside British Centurion, Churchill, and Cromwell tanks.

The Incheon landings on 15 September cut off the KPA supply lines, causing their armored forces and infantry to run out of fuel, ammunition, and other supplies. As a result of this and the Pusan perimeter breakout, the KPA had to retreat, and many of the T-34s and heavy weapons had to be abandoned. By the time the KPA withdrew from the South, 239 T-34s, and 74 SU-76 self-propelled guns were lost. After November 1950, KPA armor was rarely encountered. Following the initial assault by the North, the Korean War saw limited use of tanks and featured no large-scale tank battles. The mountainous, forested terrain, especially in the eastern central zone, was poor tank country, limiting their mobility. Through the last two years of the war in Korea, UN tanks served largely as infantry support and mobile artillery pieces.

Following the North Korean invasion, the UN Command (UNC) began an extensive bombing campaign against North Korea that lasted until the end of the war in July 1953. It was the first major bombing campaign for the US Air Force (USAF) since its inception in 1947 from the US Army Air Forces (USAAF). The air-to-ground attacks "ranged from the widespread and continual use of firebombing to threats to use nuclear and chemical weapons, finally to the destruction of huge North Korean dams in the last stages of the war."

The first bombing attack on North Korea was approved on the fourth day of the war MacArthur immediately upon request by the Commanding General of the Far East Air Forces (FEAF), George E. Stratemeyer. Massive incendiary attacks began in late July. US airpower conducted 7,000 close support and interdiction airstrikes that month, which helped slow the North Korean rate of advance to 3 kilometres (2 miles) per day.

North Korea ranks as among the most heavily bombed countries in history, and the United States dropped a total of 635,000 tons of bombs (including 32,557 tons of napalm) on Korea, more than during the entire Pacific War. Almost every substantial building in North Korea was destroyed during the war. The war's highest-ranking US prisoner-of-war, Major General William F. Dean, reported that the majority of North Korean cities and villages he saw were either rubble or snow-covered wasteland. Dean Rusk, a supporter of the war and later US Secretary of State, said the United States bombed "everything that moved in North Korea, every brick standing on top of another". Hydroelectric and irrigation dams were bombed in the later stages of the war, flooding farmland and destroying crops. North Korean factories, schools, hospitals, and government offices were forced to move underground, and air defenses were "non-existent".

According to a Chosun Ilbo report in 2001, a report by Soviet ambassador and chief military adviser to North Korea, Lieutenant General V. N. Razuvaev, estimated 282,000 North Korean deaths in bombing raids during the war. In an interview with Air Force historians in 1988, USAF General Curtis LeMay commented on efforts to win the war as a whole, including the strategic bombing campaign, saying "Right at the start of the war, unofficially, I slipped a message in 'under the carpet' in the Pentagon that we ought to turn SAC lose with some incendiaries on some North Korean towns. The answer came back, under the carpet again, that there would be too many civilian casualties; we couldn't do anything like that. We went over there and fought the war and eventually burned down every town in North Korea anyway, some way or another, and some in South Korea, too ... Over a period of three years or so we killed off, what, 20 percent of the population of Korea, as direct casualties of war or from starvation and exposure? Over a period of three years, this seemed to be acceptable to everybody, but to kill a few people at the start right away, no, we can't seem to stomach that."

General Matthew Ridgway said that except for air power, "the war would have been over in 60 days with all Korea in Communist hands". UN Command air forces flew 1,040,708 combat and combat support sorties during the Korean War. The FEAF flew the majority of sorties at 710,886 (69.3% of sorties), with the US Navy performing 16.1%, the US Marine Corps 10.3%, and 4.3% by other allied air forces.

Over the course of the war, USAF B-29 Superfortress heavy bombers flew 20,000 sorties and dropped 220,000 tons in Korea. At first, the bomber was used in strategic day-bombing missions, although North Korea's few strategic targets and industries were quickly destroyed. Future B-29 bombing raids were then restricted to night missions, justified as largely in a supply-interdiction role. The B-29 Superfortresses dropped the VB-3 "Razon" (1,000-pound explosives) and the VB-13 "Tarzon" (12,000-pound bombs) in Korea, mostly for demolishing major bridges, like the ones across the Yalu River, and for attacks on dams. The aircraft also was used for numerous leaflet drops in North Korea, such as those for Operation Moolah.

A Superfortress of the 91st Strategic Reconnaissance Squadron flew the last B-29 mission of the war on 27 July 1953.

The Korean War was the first in which jet aircraft played the central role in air combat. Once-formidable fighters such as the P-51 Mustang, F4U Corsair, and Hawker Sea Fury — all piston-engined, propeller-driven, and designed during World War II — relinquished their air-superiority roles to a new generation of faster, jet-powered fighters arriving in the theater. For the initial months of the war, the P-80 Shooting Star, F9F Panther, Gloster Meteor, and other jets under the UN flag dominated the Korean People's Air Force (KPAF) propeller-driven Soviet Yakovlev Yak-9, and Lavochkin La-9s. By early August 1950, the KPAF was reduced to only about 20 planes.

Chinese intervention in late October 1950 bolstered the KPAF with the MiG-15, one of the world's most advanced jet fighters. The USAF countered the MiG-15 by sending over three squadrons of its most capable fighter, the F-86 Sabre, which arrived in December 1950. The Soviet Union denied the involvement of their personnel in anything other than an advisory role, but air combat quickly resulted in Soviet pilots dropping their code signals and speaking over the radio in Russian. This known direct Soviet participation was a casus belli that the UN Command deliberately overlooked, lest the war expand to include the Soviet Union and potentially escalate into atomic warfare.

After the Korean War and to the present day, the US Air Force reported an inflated F-86 Sabre kill ratio in excess of 10:1, with 792 MiG-15s and 108 other aircraft shot down by Sabres, and 78 Sabres lost to enemy fire. The Soviet Air Force reported some 1,100 air-to-air victories and 335 MiG combat losses, while China's PLAAF reported 231 combat losses, mostly MiG-15s, and 168 other aircraft lost. The KPAF reported no data, but the UN Command estimates some 200 KPAF aircraft lost in the war's first stage, and 70 additional aircraft after the Chinese intervention. The USAF disputes Soviet and Chinese claims of 650 and 211 downed F-86s, respectively. More modern American estimates place the overall USAF kill ratio at around 1.8:1 with the ratio dropping to 1.3:1 against MiG-15s with Soviet pilots but increasing to a dominant 12:1 against Chinese and North Korean adversaries.

Reports by Lieutenant General Sidor Slyusarev, commander of Soviet air forces in Korea, are more favorable to the communist side. The 64th Corps claimed a total 1,097 enemy aircraft of all types during operations, for the loss of 335 aircraft (including lost to enemy ground fire, accidents, etc.) and 110 pilots. Soviet reports put the overall kill ratio at 3.4:1 in favor of Soviet pilots. As reported, effectiveness of the Soviet fighters declined as the war progressed. from an overall kill ratio of 7.9:1 from November 1950 through January 1952, declining to 2.2:1 in later 1952 and 1.9:1 in 1953. This was because more advanced jet fighters appeared on the UN side as well as improved US tactics.

Regardless of the actual ratio, American Sabres were very effective at controlling the skies over Korea. Since no other UN fighter could contend with the MiG-15, F-86s largely took over air combat once they arrived, relegating other aircraft to air-to-ground operations. Despite much greater numbers (the number of Sabres in theater never exceeded 150 while MiG-15s reached 900 at their peak), communist aircraft were seldom encountered south of Pyongyang. UN ground forces, supply lines, and infrastructure were not attacked from the air. Although North Korea had 75 airfields capable of supporting MiGs, after 1951, any serious effort to operate from them was abandoned. The MiGs were instead based across the Yalu River in the safety of China. This confined most air-to-air engagements to MiG Alley. UN aircraft had free rein to conduct strike missions over enemy territory with little fear of interception. Although jet dogfights are remembered as a prominent part of the Korean War, counter-air missions comprised just 12% of Far East Air Forces sorties, and four times as many sorties were performed for close air support and interdiction.

The war marked a major milestone not only for fixed-wing aircraft, but also for rotorcraft, featuring the first large-scale deployment of helicopters for medical evacuation (medevac). In 1944–45, during World War II, the YR-4 helicopter had seen limited ambulance duty. In Korea, where rough terrain prevented use of the jeep as a speedy medevac vehicle, helicopters like the Sikorsky H-19 were heavily used. This helped reduce fatal casualties to a dramatic degree when combined with complementary medical innovations such as Mobile Army Surgical Hospitals (MASH). As such, the medical evacuation and care system for the wounded was so effective for the UN forces that a wounded soldier who arrived at a MASH unit alive typically had a 97% chance of survival. The limitations of jet aircraft for close air support highlighted the helicopter's potential in the role, leading to the development of the helicopter gunships used in the Vietnam War.

Because neither Korea had a significant navy, the war featured few naval battles. A skirmish between North Korea and the UN Command occurred on 2 July 1950; the US Navy cruiser USS Juneau, the Royal Navy cruiser HMS Jamaica and the Royal Navy frigate HMS Black Swan fought four North Korean torpedo boats and two mortar gunboats, and sank them. USS Juneau later sank several ammunition ships that had been present. The last sea battle of the Korean War occurred days before the Battle of Incheon; the ROK ship PC-703 sank a North Korean minelayer in the Battle of Haeju Island, near Incheon.

About 3 million people were killed in the war, mostly civilians, making it perhaps the deadliest conflict of the Cold War era. Samuel Kim lists the Korean War as the deadliest conflict in East Asia, and the region most affected by armed conflict related to the Cold War.

Though only rough estimates of civilian fatalities are available, scholars have noted that the percentage of civilian casualties in Korea was higher than in World War II or the Vietnam War, with historian Bruce Cumings putting civilian casualties at 2 million and Guenter Lewy in the range of 2–3 million. According to one source, 1,658,436 civilians died, meaning a 74% civilian death rate.

Cumings states that civilians represent at least half the war's casualties, while Lewy suggests it may have gone as high as 70%, compared to his estimates of 42% in World War II and 30 to 46% in Vietnam. Data compiled by the Peace Research Institute Oslo lists just under 1 million battle deaths over the war and a mid-estimate of 3 million total deaths, attributing the difference to excess mortality among civilians from one-sided massacres, starvation, and disease. Compounding this devastation for civilians, virtually all major cities on the Peninsula were destroyed. In per capita and absolute terms, North Korea was the most devastated by the war. According to Charles K. Armstrong, the war resulted in the death of an estimated 12 to 15% of the North Korean population (c. 10 million), "a figure close to or surpassing the proportion of Soviet citizens killed in World War II".

According to the South Korean Ministry of National Defense, there were over 750,000 confirmed violent civilian deaths during the war, another million civilians were pronounced missing, and millions more ended up as refugees. The UN Command estimated that 5 million Koreans (both north and south) had left their homes and become refugees (including internally displaced) by the summer of 1951. Over 1.5 million North Koreans fled to the South.

South Korea originally reported 137,899 military deaths and 24,495 missing, 450,742 wounded, and 8,343 prisoners. The South Korean government later stated an official tally of 184,573 killed or missing in action, with about 257,000 military deaths in total from all causes. Historian Max Hastings estimates South Korean casualties higher at 415,000 killed and 429,000 wounded.

The United States suffered 33,739 deaths in battle, including those missing and declared dead, and 2,835 non-battle deaths. There were 17,730 other non-battle US military deaths outside Korea during the same period erroneously included as war deaths until 2000. The US government estimates that 103,284 American soldiers were wounded in action, with another 10,218 that were captured by Communist forces or went missing in action. 1,789,000 American soldiers served in the war, accounting for 31% of the 5,720,000 Americans who served on active duty worldwide from 1950 to 1953.

American combat casualties were over 90% of non-Korean UN losses. US battle deaths were 8,516 up to their first engagement with the Chinese on 1 November 1950. The first four months prior to the Chinese intervention were by far the bloodiest per day for US forces, as they engaged the well-equipped KPA in intense fighting. American medical records show that from July to October 1950, the army sustained 31% of the combat deaths it ultimately incurred in the entire 37-month war. 45% of American casualties would be incurred after the first armistice negotiations with the Communists took place.

Deaths from non-American militaries for the UN Command totaled 3,730, with another 379 missing.

The Chinese and North Koreans claimed that about 390,000 soldiers from the US, 660,000 soldiers from South Korea and 29,000 other UN soldiers were "eliminated" from the battlefield.

Chinese sources reported that the PVA suffered 114,000 battle deaths, 21,000 deaths from wounds, 13,000 deaths from illness, 340,000 wounded, and 7,600 missing. 7,110 Chinese POWs were repatriated to China. In 2010, the Chinese government revised its official tally of war losses to 183,108 dead (114,084 in combat, 70,000 deaths from wounds, illness and other causes) and 25,621 missing, a total of 208,729 dead or missing. In 2020, Chinese state media outlet China Daily reported the number of Chinese soldiers killed in the war at 197,653. Chinese soldiers who served in Korea faced a greater chance of being killed or wounded than those who served in World War II or the Chinese Civil War.

According to the South Korean Ministry of National Defense, North Korean military losses totaled 294,151 dead, 91,206 missing, and 229,849 wounded, giving North Korea the highest military deaths of any belligerent in absolute and relative terms. The PRIO Battle Deaths Dataset gave a similar figure for North Korean military deaths of 316,579. Chinese sources reported similar figures for the North Korean military of 290,000 "casualties" and 90,000 captured. The financial cost for North Korea was massive indirect losses and lost economic activity; the country was devastated by the cost of the war and the American bombing campaign, which, among other things, destroyed 85% of North Korea's buildings and 95% of its power generation. The Soviet Union suffered 299 dead, with 335 planes lost.

The Americans claimed during the war that they had killed over 1,500,000 Chinese and North Korean soldiers. Historian Max Hastings states that this figure is likely exaggerated, with a more realistic number being around 500,000 dead Chinese soldiers. Historian Hans van de Ven writes that reasonable estimates for Chinese and North Korean military casualties are roughly half a million casualties for each of their respective armies, or one million total.

The American Department of Defense estimated the PVA suffered about 400,000 killed and 486,000 wounded, while the KPA suffered 215,000 killed, 303,000 wounded, and over 101,000 captured or missing. Cumings claims a much higher figure of 900,000 dead Chinese soldiers.

Overall, 73% of Chinese infantry troops served in Korea (25 of 34 armies, or 79 of 109 infantry divisions, were rotated in). More than 52% of the Chinese air force, 55% of the tank units, 67% of the artillery divisions, and 100% of the railroad engineering divisions were sent to Korea as well. China spent over 10 billion yuan on the war (roughly US$3.3 billion), not counting USSR aid. This included US$1.3 billion in money owed to the Soviet Union by the end of it. This was a relatively large cost, as China had only 4% of the national income of the United States. Spending on the war constituted 34–43% of China's annual government budget from 1950 to 1953, depending on the year. Despite its underdeveloped economy, Chinese military spending was the world's fourth largest globally for the war's duration after that of the US, the Soviet Union, and the UK.

There were numerous atrocities and massacres of civilians throughout the war committed by all sides, starting in its first days, though the overwhelming majority of such atrocities have been attributed to acts by the South and the US.

In 2005–2010, a South Korean Truth and Reconciliation Commission investigated atrocities and other human rights violations through much of the 20th century, from the Japanese colonial period through the Korean War and beyond. It excavated some mass graves from the Bodo League massacres and confirmed the general outlines of those political executions. Of the Korean War-era massacres the commission was petitioned to investigate, 82% were perpetrated by South Korean forces, with 18% perpetrated by North Korean forces. The Bodo League massacres were the mass killings of alleged communists by South Korean forces in the summer of 1950 following North Korea's invasion. The commission was able to verify 4,934 execution victims in these massacres but historians estimate that at least 60,000 people were killed with estimates reaching 200,000. The most well known North Korean massacre was the Seoul National University Hospital massacre on 28 June 1950 where over 1,000 patients and wounded soldiers were killed by North Korean soldiers.

North Korean soldiers and officials systematically murdered South Korean civilians throughout the war, as well as conducting a campaign of mass kidnapping and illegal impressment of civilians in occupied territory. The massacres committed by North Korea were sometimes carried out through public executions using "people's trials" and other times through sporadic bursts of violence by occupying soldiers. The number of victims is estimated in the hundreds of thousands. The Truth and Reconciliation Commission established a minimum of 129,000 South Korean civilians killed by the KPA. Additionally, of the 70,000 South Korean prisoners of war held by the North Koreans, only 8,000 were ever returned. R. J. Rummel estimated that North Korean democide killed 363,000 South Koreans during the war.

The commission also received petitions alleging more than 200 large-scale killings of South Korean civilians by the US military during the war, mostly air attacks. It confirmed several such cases, including refugees crowded into a cave attacked with napalm bombs, which survivors said killed 360 people, and an air attack that killed 197 refugees gathered in a field in the far south. It recommended South Korea seek reparations from the United States, but in 2010, a reorganized commission under a new, conservative government concluded that most US mass killings resulted from "military necessity", while in a small number of cases, they concluded, the US military had acted with "low levels of unlawfulness", but the commission recommended against seeking reparations.

The UN bombing of North Korea during the Korean War has been condemned as a war crime by various authors. A Soviet report estimated that 282,000 North Korean civilians were killed in the bombing. Historian Bruce Cumings has stated: "What hardly any Americans know or remember is that we carpet-bombed the North for three years with next to no concern for civilian casualties." Scholar Sahr Conway-Lanz has stated that, "During the war, American military and civilian officials stretched the term 'military target' to include virtually all human-made structures, capitalizing on the vague distinction between the military and civilian segments of an enemy society. They came to apply the logic of total war to the destruction of the civil infrastructure in North Korea." Cumings wrote "The United Nations Genocide Convention defined the term as acts committed 'with intent to destroy, in whole or in part, a national, ethnically, racial or religious group'. This would include deliberately inflicting on the group conditions of life calculated to bring about its physical destruction in whole or in part. It was approved in 1948 and entered into force in 1951—just as the USAF was inflicting genocide, under this definition and under the aegis of the United Nations Command, on the citizens of North Korea." Cumings has been accused of engaging in apologia for North Korea, exaggerating American crimes, denying North Korean responsibility for starting the war, and whitewashing the conditions of the country's kwanliso concentration camps.

In October 1950, 25 unnamed North Korean prisoners of war were chosen by the U.S. Government as the first test subjects in a planned series of interrogation and brainwashing experiments which eventually became known as Project Artichoke. Korean POWs were subjected to hypnosis experiments involving forced addiction and withdrawals from psychoactive substances, such as LSD. Prisoners were also unwittingly sedated and purposely infected with diseases such as dengue fever, to determine if said diseases could be employed as Incapacitating agents.

Almost a half century after the Korean War, on 29 September 1999, a team of Associated Press journalists reported that a dozen ex-American GI soldiers had confirmed that US forces killed large numbers of civilian refugees – up to 400, mostly women, children and elderly people – in July 1950 under and around a railroad trestle in the hamlet of No Gun Ri. The AP "also published declassified military documents showing that US military commanders had issued standing orders to shoot civilians". The highest levels of the South Korean and US governments ordered and conducted lengthy investigations on the matter. "The Pentagon's No Gun Ri review spent a year examining over a million documents and the interview statements from approximately 275 American and Korean witnesses and issued its findings in early 2001." In its final report, after years of dismissing the story of Korean villagers, the US Army affirmed the AP's finding that American troops had killed the refugees.