World War I

Before World War II, the events of 1914–1918 were commonly referred to as the Great War or simply the World War. In August 1914, The Independent stated of the conflict, "This is the Great War. It names itself." In the decade after its end, many hoped it would be "the war to end all wars," because of its unprecedented destructiveness and death toll. The earliest known use of the term First World War appeared in September 1914, when German biologist and philosopher Ernst Haeckel wrote that the ongoing "European War" would become "the first world war in the full sense of the word."

For much of the 19th century, the major European powers maintained a tenuous balance of power, known as the Concert of Europe. After 1848, this was challenged by Britain's withdrawal into splendid isolation, the decline of the Ottoman Empire, New Imperialism, and the rise of Prussia under Otto von Bismarck. Victory in the 1870–1871 Franco-Prussian War allowed Bismarck to consolidate a German Empire. After 1871, French policy aimed to avenge this defeat and expand France's colonial empire.

In 1873, Bismarck negotiated the League of the Three Emperors, which included Austria-Hungary, Russia, and Germany. After the 1877–1878 Russo-Turkish War, the League was dissolved due to Austrian concerns over the expansion of Russian influence in the Balkans, an area they considered to be of vital strategic interest. Germany and Austria-Hungary then formed the 1879 Dual Alliance, which became the Triple Alliance when Italy joined in 1882. For Bismarck, the purpose of these agreements was to isolate France by ensuring the three empires resolved any disputes among themselves. In 1887, Bismarck set up the Reinsurance Treaty, a secret agreement between Germany and Russia to remain neutral if either were attacked by France or Austria-Hungary.

For Bismarck, peace with Russia was the foundation of German foreign policy, but in 1890, he was forced to retire by Wilhelm II. The latter was persuaded not to renew the Reinsurance Treaty by his new Chancellor, Leo von Caprivi. This gave France an opening to agree to the Franco-Russian Alliance in 1894, which was then followed by the 1904 Entente Cordiale with Britain. The Triple Entente was completed by the 1907 Anglo-Russian Convention. While not formal alliances, by settling longstanding colonial disputes in Asia and Africa, British support for France or Russia in any future conflict became a possibility. This was accentuated by British and Russian support for France against Germany during the 1911 Agadir Crisis.

German economic and industrial strength continued to expand rapidly post-1871. Backed by Wilhelm II, Admiral Alfred von Tirpitz sought to use this growth to build an Imperial German Navy that could compete with the British Royal Navy. This policy was based on the work of US naval author Alfred Thayer Mahan, who argued that possession of a blue-water navy was vital for global power projection; Tirpitz had his books translated into German, while Wilhelm made them required reading for his advisors and senior military personnel.

Bismarck opposed any attempt to compete with the Royal Navy, since he believed Britain would not interfere in Europe as long as its maritime supremacy remained secure. His dismissal in 1890 led to a change in policy and the start of an Anglo-German naval arms race. Despite the vast sums spent by Tirpitz, the launch of HMS Dreadnought in 1906 made every existing battleship obsolete, and gave the British a technological advantage they never relinquished. Ultimately, Germany invested huge resources on creating a navy large enough to antagonise Britain, but not defeat it. In 1911, Chancellor Theobald von Bethmann Hollweg acknowledged defeat, leading to the Rüstungswende or 'armaments turning point', when he switched expenditure from the navy to the army.

This decision was driven by German concerns over the speed of Russia's recovery from defeat in the Russo-Japanese War and the subsequent 1905 Russian Revolution. Economic reforms led to a significant post-1908 expansion of railways and transportation infrastructure, particularly in its western border regions. Since Germany and Austria-Hungary relied on faster mobilisation to compensate for their numerical inferiority compared to Russia, the threat posed by the closing of this gap was more important than competing with the Royal Navy. After Germany expanded its standing army by 170,000 troops in 1913, France extended compulsory military service from two to three years, sparking similar measures from the Balkan powers, Italy, the Ottoman Empire and Austria-Hungary. Absolute figures are difficult to calculate due to differences in categorising expenditure since they often omit civilian infrastructure projects like railways which had logistical importance and military use. However, from 1908 to 1913, military spending by the six major European powers increased by over 50% in real terms.

The years before 1914 were marked by a series of crises in the Balkans, as other powers sought to benefit from the Ottoman decline. While Pan-Slavic and Orthodox Russia considered itself the protector of Serbia and other Slav states, they preferred the strategically vital Bosporus straits to be controlled by a weak Ottoman government, rather than an ambitious Slav power like Bulgaria. Russia had ambitions in northeastern Anatolia while its clients had overlapping claims in the Balkans. These competing interests divided Russian policy-makers and added to regional instability.

Austrian statesmen viewed the Balkans as essential for the continued existence of their Empire and saw Serbian expansion as a direct threat. The 1908–1909 Bosnian Crisis began when Austria annexed the former Ottoman territory of Bosnia and Herzegovina, which it had occupied since 1878. Timed to coincide with the Bulgarian Declaration of Independence from the Ottoman Empire, this unilateral action was denounced by the European powers, but accepted as there was no consensus on how to resolve the situation. Some historians see this as a significant escalation, ending any chance of Austria cooperating with Russia in the Balkans, while also damaging diplomatic relations between Serbia and Italy.

Tensions increased after the 1911–1912 Italo-Turkish War demonstrated Ottoman weakness and led to the formation of the Balkan League, an alliance of Serbia, Bulgaria, Montenegro, and Greece. The League quickly overran most of the Ottomans' territory in the Balkans during the 1912–1913 First Balkan War, much to the surprise of outside observers. The Serbian capture of ports on the Adriatic resulted in partial Austrian mobilisation, starting on 21 November 1912, including units along the Russian border in Galicia. The Russian government decided not to mobilise in response, unprepared to precipitate a war.

The great powers sought to re-assert control through the 1913 Treaty of London, which had created an independent Albania while enlarging the territories of Bulgaria, Serbia, Montenegro and Greece. However, disputes between the victors sparked the 33-day Second Balkan War, when Bulgaria attacked Serbia and Greece on 16 June 1913; it was defeated, losing most of Macedonia to Serbia and Greece, and Southern Dobruja to Romania. The result was that even countries which benefited from the Balkan Wars, such as Serbia and Greece, felt cheated of their "rightful gains", while for Austria it demonstrated the apparent indifference with which other powers viewed their concerns, including Germany. This complex mix of resentment, nationalism and insecurity helps explain why the pre-1914 Balkans became known as the "powder keg of Europe".

On 28 June 1914, Archduke Franz Ferdinand of Austria, heir presumptive to Emperor Franz Joseph I of Austria, visited Sarajevo, the capital of the recently annexed Bosnia and Herzegovina. Cvjetko Popović, Gavrilo Princip, Nedeljko Čabrinović, Trifko Grabež, Vaso Čubrilović (Bosnian Serbs) and Muhamed Mehmedbašić (from the Bosniaks community), from the movement known as Young Bosnia, took up positions along the Archduke's motorcade route, to assassinate him. Supplied with arms by extremists within the Serbian Black Hand intelligence organisation, they hoped his death would free Bosnia from Austrian rule.

Čabrinović threw a grenade at the Archduke's car and injured two of his aides. The other assassins were also unsuccessful. An hour later, as Ferdinand was returning from visiting the injured officers in hospital, his car took a wrong turn into a street where Gavrilo Princip was standing. He fired two pistol shots, fatally wounding Ferdinand and his wife Sophie.

According to historian Zbyněk Zeman, in Vienna "the event almost failed to make any impression whatsoever. On 28 and 29 June, the crowds listened to music and drank wine, as if nothing had happened." Nevertheless, the impact of the murder of the heir to the throne was significant, and has been described by historian Christopher Clark as a "9/11 effect, a terrorist event charged with historic meaning, transforming the political chemistry in Vienna".

Austro-Hungarian authorities encouraged subsequent anti-Serb riots in Sarajevo. Violent actions against ethnic Serbs were also organised outside Sarajevo, in other cities in Austro-Hungarian-controlled Bosnia and Herzegovina, Croatia and Slovenia. Austro-Hungarian authorities in Bosnia and Herzegovina imprisoned approximately 5,500 prominent Serbs, 700 to 2,200 of whom died in prison. A further 460 Serbs were sentenced to death. A predominantly Bosniak special militia known as the Schutzkorps was established, and carried out the persecution of Serbs.

The assassination initiated the July Crisis, a month of diplomatic manoeuvring among Austria-Hungary, Germany, Russia, France and Britain. Believing that Serbian intelligence helped organise Franz Ferdinand's murder, Austrian officials wanted to use the opportunity to end Serbian interference in Bosnia and saw war as the best way of achieving this. However, the Foreign Ministry had no solid proof of Serbian involvement. On 23 July, Austria delivered an ultimatum to Serbia, listing ten demands made intentionally unacceptable to provide an excuse for starting hostilities.

Serbia ordered general mobilisation on 25 July, but accepted all the terms, except for those empowering Austrian representatives to suppress "subversive elements" inside Serbia, and take part in the investigation and trial of Serbians linked to the assassination. Claiming this amounted to rejection, Austria broke off diplomatic relations and ordered partial mobilisation the next day; on 28 July, they declared war on Serbia and began shelling Belgrade. Russia ordered general mobilisation in support of Serbia on 30 July.

Anxious to ensure backing from the SPD political opposition by presenting Russia as the aggressor, German Chancellor Bethmann Hollweg delayed the commencement of war preparations until 31 July. That afternoon, the Russian government were handed a note requiring them to "cease all war measures against Germany and Austria-Hungary" within 12 hours. A further German demand for neutrality was refused by the French who ordered general mobilisation but delayed declaring war. The German General Staff had long assumed they faced a war on two fronts; the Schlieffen Plan envisaged using 80% of the army to defeat France, then switching to Russia. Since this required them to move quickly, mobilisation orders were issued that afternoon. Once the German ultimatum to Russia expired on the morning of 1 August, the two countries were at war.

At a meeting on 29 July, the British cabinet had narrowly decided its obligations to Belgium under the 1839 Treaty of London did not require it to oppose a German invasion with military force; however, Prime Minister Asquith and his senior Cabinet ministers were already committed to supporting France, the Royal Navy had been mobilised, and public opinion was strongly in favour of intervention. On 31 July, Britain sent notes to Germany and France, asking them to respect Belgian neutrality; France pledged to do so, but Germany did not reply. Aware of German plans to attack through Belgium, French Commander-in-Chief Joseph Joffre asked his government for permission to cross the border and pre-empt such a move. To avoid violating Belgian neutrality, he was told any advance could come only after a German invasion. Instead, the French cabinet ordered its Army to withdraw 10 km behind the German frontier, to avoid provoking war. On 2 August, Germany occupied Luxembourg and exchanged fire with French units when German patrols entered French territory; on 3 August, they declared war on France and demanded free passage across Belgium, which was refused. Early on the morning of 4 August, the Germans invaded, and Albert I of Belgium called for assistance under the Treaty of London. Britain sent Germany an ultimatum demanding they withdraw from Belgium; when this expired at midnight, without a response, the two empires were at war.

Germany promised to support Austria-Hungary's invasion of Serbia, but interpretations of what this meant differed. Previously tested deployment plans had been replaced early in 1914, but those had never been tested in exercises. Austro-Hungarian leaders believed Germany would cover its northern flank against Russia.

Beginning on 12 August, the Austrians and Serbs clashed at the battles of the Cer and Kolubara; over the next two weeks, Austrian attacks were repulsed with heavy losses. As a result, Austria had to keep sizeable forces on the Serbian front, weakening their efforts against Russia. Serbia's victory against Austria-Hungary in the 1914 invasion has been called one of the major upset victories of the twentieth century. In 1915, the campaign saw the first use of anti-aircraft warfare after an Austrian plane was shot down with ground-to-air fire, as well as the first medical evacuation by the Serbian army.

Upon mobilisation, in accordance with the Schlieffen Plan, 80% of the German Army was located on the Western Front, with the remainder acting as a screening force in the East. Rather than a direct attack across their shared frontier, the German right wing would sweep through the Netherlands and Belgium, then swing south, encircling Paris and trapping the French army against the Swiss border. The plan's creator, Alfred von Schlieffen, head of the German General Staff from 1891 to 1906, estimated that this would take six weeks, after which the German army would transfer to the East and defeat the Russians.

The plan was substantially modified by his successor, Helmuth von Moltke the Younger. Under Schlieffen, 85% of German forces in the west were assigned to the right wing, with the remainder holding along the frontier. By keeping his left-wing deliberately weak, he hoped to lure the French into an offensive into the "lost provinces" of Alsace-Lorraine, which was the strategy envisaged by their Plan XVII. However, Moltke grew concerned that the French might push too hard on his left flank and as the German Army increased in size from 1908 to 1914, he changed the allocation of forces between the two wings to 70:30. He also considered Dutch neutrality essential for German trade and cancelled the incursion into the Netherlands, which meant any delays in Belgium threatened the viability of the plan. Historian Richard Holmes argues that these changes meant the right wing was not strong enough to achieve decisive success.

The initial German advance in the West was very successful. By the end of August, the Allied left, which included the British Expeditionary Force (BEF), was in full retreat, and the French offensive in Alsace-Lorraine was a disastrous failure, with casualties exceeding 260,000. German planning provided broad strategic instructions while allowing army commanders considerable freedom in carrying them out at the front, but Alexander von Kluck used this freedom to disobey orders, opening a gap between the German armies as they closed on Paris. The French army, reinforced by the British expeditionary corps, seized this opportunity to counter-attack and pushed the German army 40 to 80 km back. Both armies were then so exhausted that no decisive move could be implemented, so they settled in trenches, with the vain hope of breaking through as soon as they could build local superiority.

In 1911, the Russian Stavka agreed with the French to attack Germany within fifteen days of mobilisation, ten days before the Germans had anticipated, although it meant the two Russian armies that entered East Prussia on 17 August did so without many of their support elements.

By the end of 1914, German troops held strong defensive positions inside France, controlled the bulk of France's domestic coalfields, and inflicted 230,000 more casualties than it lost itself. However, communications problems and questionable command decisions cost Germany the chance of a decisive outcome, while it had failed to achieve the primary objective of avoiding a long, two-front war. As was apparent to several German leaders, this amounted to a strategic defeat; shortly after the First Battle of the Marne, Crown Prince Wilhelm told an American reporter "We have lost the war. It will go on for a long time but lost it is already."

On 30 August 1914, New Zealand occupied German Samoa (now Samoa). On 11 September, the Australian Naval and Military Expeditionary Force landed on the island of New Britain, then part of German New Guinea. On 28 October, the German cruiser SMS Emden sank the Russian cruiser Zhemchug in the Battle of Penang. Japan declared war on Germany before seizing territories in the Pacific, which later became the South Seas Mandate, as well as German Treaty ports on the Chinese Shandong peninsula at Tsingtao. After Vienna refused to withdraw its cruiser SMS Kaiserin Elisabeth from Tsingtao, Japan declared war on Austria-Hungary, and the ship was scuttled in November 1914. Within a few months, Allied forces had seized all German territories in the Pacific, leaving only isolated commerce raiders and a few holdouts in New Guinea.

Some of the first clashes of the war involved British, French, and German colonial forces in Africa. On 6–7 August, French and British troops invaded the German protectorates of Togoland and Kamerun. On 10 August, German forces in South-West Africa attacked South Africa; sporadic and fierce fighting continued for the rest of the war. The German colonial forces in German East Africa, led by Colonel Paul von Lettow-Vorbeck, fought a guerrilla warfare campaign and only surrendered two weeks after the armistice took effect in Europe.

Before the war, Germany had attempted to use Indian nationalism and pan-Islamism to its advantage, a policy continued post-1914 by instigating uprisings in India, while the Niedermayer–Hentig Expedition urged Afghanistan to join the war on the side of Central Powers. However, contrary to British fears of a revolt in India, the outbreak of the war saw a reduction in nationalist activity. Leaders from the Indian National Congress and other groups believed support for the British war effort would hasten Indian Home Rule, a promise allegedly made explicit in 1917 by Edwin Montagu, the Secretary of State for India.

In 1914, the British Indian Army was larger than the British Army itself, and between 1914 and 1918 an estimated 1.3 million Indian soldiers and labourers served in Europe, Africa, and the Middle East. In all, 140,000 soldiers served on the Western Front and nearly 700,000 in the Middle East, with 47,746 killed and 65,126 wounded. The suffering engendered by the war, as well as the failure of the British government to grant self-government to India afterward, bred disillusionment, resulting in the campaign for full independence led by Mahatma Gandhi.

Pre-war military tactics that had emphasised open warfare and individual riflemen proved obsolete when confronted with conditions prevailing in 1914. Barbed wire, machine guns and artillery allowed the creation of strong defensive systems that made massed infantry advances extremely difficult. Both sides struggled to develop tactics for breaching entrenched positions without heavy casualties. In time, the tank helped to make the front more mobile.

After the First Battle of the Marne in September 1914, Allied and German forces unsuccessfully tried to outflank each other, a series of manoeuvres later known as the "Race to the Sea". By the end of 1914, the opposing forces confronted each other along an uninterrupted line of entrenched positions from the Channel to the Swiss border. Since the Germans were normally able to choose where to stand, they generally held the high ground, while their trenches tended to be better built; those constructed by the French and English were initially considered "temporary", only needed until an offensive would destroy the German defences. Both sides tried to break the stalemate using scientific and technological advances. On 22 April 1915, at the Second Battle of Ypres, the Germans (violating the Hague Convention) used chlorine gas for the first time on the Western Front.

In February 1916, the Germans attacked French defensive positions at the Battle of Verdun, lasting until December 1916. Casualties were greater for the French, but the Germans bled heavily as well, with anywhere from 700,000 to 975,000 casualties between the two combatants. Verdun became a symbol of French determination and self-sacrifice.

The Battle of the Somme was an Anglo-French offensive from July to November 1916. The opening day, on 1 July 1916, was the bloodiest single day in the history of the British Army, which suffered 57,500 casualties, including 19,200 dead. As a whole, the Somme offensive led to an estimated 420,000 British casualties, along with 200,000 French and 500,000 Germans. The diseases that emerged in the trenches were a major killer on both sides. The living conditions led to disease and infection, such as trench foot, lice, typhus, trench fever, and the 'Spanish flu'.

At the start of the war, German cruisers were scattered across the globe, some of which were subsequently used to attack Allied merchant shipping. These were systematically hunted down by the Royal Navy, though not before causing considerable damage. One of the most successful was the SMS Emden, part of the German East Asia Squadron stationed at Qingdao, which seized or sank 15 merchantmen, a Russian cruiser and a French destroyer. Most of the squadron was returning to Germany when it sank two British armoured cruisers at the Battle of Coronel in November 1914, before being virtually destroyed at the Battle of the Falkland Islands in December. The SMS Dresden escaped with a few auxiliaries, but after the Battle of Más a Tierra, these too were either destroyed or interned.

Soon after the outbreak of hostilities, Britain began a naval blockade of Germany. This proved effective in cutting off vital supplies, though it violated accepted international law. Britain also mined international waters which closed off entire sections of the ocean, even to neutral ships. Since there was limited response to this tactic, Germany expected a similar response to its unrestricted submarine warfare.

The Battle of Jutland in May/June 1916 was the only full-scale clash of battleships during the war, and one of the largest in history. The clash was indecisive, though the Germans inflicted more damage than they received; thereafter the bulk of the German High Seas Fleet was confined to port.

German U-boats attempted to cut the supply lines between North America and Britain. The nature of submarine warfare meant that attacks often came without warning, giving the crews of the merchant ships little hope of survival. The United States launched a protest, and Germany changed its rules of engagement. After the sinking of the passenger ship RMS Lusitania in 1915, Germany promised not to target passenger liners, while Britain armed its merchant ships, placing them beyond the protection of the "cruiser rules", which demanded warning and movement of crews to "a place of safety" (a standard that lifeboats did not meet). Finally, in early 1917, Germany adopted a policy of unrestricted submarine warfare, realising the Americans would eventually enter the war. Germany sought to strangle Allied sea lanes before the United States could transport a large army overseas, but, after initial successes, eventually failed to do so.

The U-boat threat lessened in 1917, when merchant ships began travelling in convoys, escorted by destroyers. This tactic made it difficult for U-boats to find targets, which significantly lessened losses; after the hydrophone and depth charges were introduced, destroyers could potentially successfully attack a submerged submarine. Convoys slowed the flow of supplies since ships had to wait as convoys were assembled; the solution was an extensive program of building new freighters.

In the aftermath of the war, the German, Austro-Hungarian, Ottoman, and Russian empires had disappeared. Numerous nations regained their former independence, and new ones were created. Four dynasties fell as a result of the war: the Romanovs, the Hohenzollerns, the Habsburgs, and the Ottomans. Belgium and Serbia were badly damaged, as was France, with 1.4 million soldiers dead, not counting other casualties. Germany and Russia were similarly affected.

A formal state of war between the two sides persisted for another seven months, until the signing of the Treaty of Versailles with Germany on 28 June 1919. The US Senate did not ratify the treaty despite public support for it, and did not formally end its involvement in the war until the Knox–Porter Resolution was signed on 2 July 1921 by President Warren G. Harding. For the British Empire, the state of war ceased under the provisions of the Termination of the Present War (Definition) Act 1918 concerning:

Germany on 10 January 1920.

Austria on 16 July 1920.

Bulgaria on 9 August 1920.

Hungary on 26 July 1921.

Turkey on 6 August 1924.

Some war memorials date the end of the war as being when the Versailles Treaty was signed in 1919, which was when many of the troops serving abroad finally returned home; by contrast, most commemorations of the war's end concentrate on the armistice of 11 November 1918.

The Paris Peace Conference imposed a series of peace treaties on the Central Powers officially ending the war. The 1919 Treaty of Versailles dealt with Germany and, building on Wilson's 14th point, established the League of Nations on 28 June 1919.

The Central Powers had to acknowledge responsibility for "all the loss and damage to which the Allied and Associated Governments and their nationals have been subjected as a consequence of the war imposed upon them by" their aggression. In the Treaty of Versailles, this statement was Article 231. This article became known as the "War Guilt Clause", as the majority of Germans felt humiliated and resentful. The Germans felt they had been unjustly dealt with by what they called the "diktat of Versailles". German historian Hagen Schulze said the Treaty placed Germany "under legal sanctions, deprived of military power, economically ruined, and politically humiliated." Belgian historian Laurence Van Ypersele emphasises the central role played by memory of the war and the Versailles Treaty in German politics in the 1920s and 1930s:

Active denial of war guilt in Germany and German resentment at both reparations and continued Allied occupation of the Rhineland made widespread revision of the meaning and memory of the war problematic. The legend of the "stab in the back" and the wish to revise the "Versailles diktat", and the belief in an international threat aimed at the elimination of the German nation persisted at the heart of German politics. Even a man of peace such as [Gustav] Stresemann publicly rejected German guilt. As for the Nazis, they waved the banners of domestic treason and international conspiracy in an attempt to galvanise the German nation into a spirit of revenge. Like a Fascist Italy, Nazi Germany sought to redirect the memory of the war to the benefit of its policies.

Meanwhile, new nations liberated from German rule viewed the treaty as a recognition of wrongs committed against small nations by much larger aggressive neighbours.

Austria-Hungary was partitioned into several successor states, largely but not entirely along ethnic lines. Czechoslovakia, Italy, Poland, Romania and Yugoslavia received territories from the Dual Monarchy (while the formerly separate and autonomous Kingdom of Croatia-Slavonia was incorporated into Yugoslavia). The details were contained in the treaties of Saint-Germain-en-Laye and Trianon. As a result, Hungary lost 64% of its total population, decreasing from 20.9 million to 7.6 million, and losing 31% (3.3 out of 10.7 million) of its ethnic Hungarians. According to the 1910 census, speakers of the Hungarian language included approximately 54% of the entire population of the Kingdom of Hungary. Within the country, numerous ethnic minorities were present: 16.1% Romanians, 10.5% Slovaks, 10.4% Germans, 2.5% Ruthenians, 2.5% Serbs, and 8% others. Between 1920 and 1924, 354,000 Hungarians fled former Hungarian territories attached to Romania, Czechoslovakia, and Yugoslavia.

Russia lost much of its western frontier as the newly independent nations of Estonia, Finland, Latvia, Lithuania, and Poland were carved from it. Romania took control of Bessarabia in April 1918.

After 123 years, Poland re-emerged as an independent country. The Kingdom of Serbia and its dynasty, as a "minor Entente nation" and the country with the most casualties per capita, became the backbone of a new multinational state, the Kingdom of Serbs, Croats and Slovenes, later renamed Yugoslavia. Czechoslovakia, combining the Kingdom of Bohemia with parts of the Kingdom of Hungary, became a new nation. Romania would unite all Romanian-speaking people under a single state, leading to Greater Romania.

In Australia and New Zealand, the Battle of Gallipoli became known as those nations' "Baptism of Fire". It was the first major war in which the newly established countries fought, and it was one of the first times that Australian troops fought as Australians, not just subjects of the British Crown, and independent national identities for these nations took hold. Anzac Day, named after the Australian and New Zealand Army Corps (ANZAC), commemorates this defining moment.

In the aftermath of World War I, Greece fought against Turkish nationalists led by Mustafa Kemal, a war that eventually resulted in a massive population exchange between the two countries under the Treaty of Lausanne. According to various sources, several hundred thousand Greeks died during this period, which was tied in with the Greek genocide.

An estimated number of military and civilian casualties in World War I was about 15 to 22 million deaths and about 23 million wounded military personnel, ranking it among the deadliest conflicts in human history. The total number of deaths includes between 9 and 11 million military personnel, with an estimated civilian death toll of about 6 to 13 million.

Of the 60 million European military personnel who were mobilised from 1914 to 1918, an estimated 8 million were killed, 7 million were permanently disabled, and 15 million were seriously injured. Germany lost 15.1% of its active male population, Austria-Hungary lost 17.1%, and France lost 10.5%. France mobilised 7.8 million men, of which 1.4 million died and 3.2 million were injured. Approximately 15,000 deployed men sustained gruesome facial injuries, causing social stigma and marginalisation; they were called the gueules cassées (broken faces). In Germany, civilian deaths were 474,000 higher than in peacetime, due in large part to food shortages and malnutrition that had weakened disease resistance. These excess deaths are estimated as 271,000 in 1918, plus another 71,000 in the first half of 1919 when the blockade was still in effect. Starvation caused by famine killed approximately 100,000 people in Lebanon.

Diseases flourished in the chaotic wartime conditions. In 1914 alone, louse-borne epidemic typhus killed 200,000 in Serbia. Starting in early 1918, a major influenza epidemic known as Spanish flu spread across the world, accelerated by the movement of large numbers of soldiers, often crammed together in camps and transport ships with poor sanitation. The Spanish flu killed at least 17 to 25 million people, including an estimated 2.64 million Europeans and as many as 675,000 Americans. Between 1915 and 1926, an epidemic of encephalitis lethargica affected nearly 5 million people worldwide.

Eight million equines, mostly horses, donkeys and mules died, three-quarters of them from the extreme conditions they worked in.

The German army was the first to successfully deploy chemical weapons during the Second Battle of Ypres (April–May 1915), after German scientists under the direction of Fritz Haber at the Kaiser Wilhelm Institute developed a method to weaponise chlorine. The use of chemical weapons had been sanctioned by the German High Command to force Allied soldiers out of their entrenched positions, complementing rather than supplanting more lethal conventional weapons. Chemical weapons were deployed by all major belligerents throughout the war, inflicting approximately 1.3 million casualties, of which about 90,000 were fatal. The use of chemical weapons in warfare was a direct violation of the 1899 Hague Declaration Concerning Asphyxiating Gases and the 1907 Hague Convention on Land Warfare, which prohibited their use.

The ethnic cleansing of the Ottoman Empire's Armenian population, including mass deportations and executions, during the final years of the Ottoman Empire is considered genocide. The Ottomans carried out organised and systematic massacres of the Armenian population at the beginning of the war and manipulated acts of Armenian resistance by portraying them as rebellions to justify further extermination. In early 1915, several Armenians volunteered to join the Russian forces and the Ottoman government used this as a pretext to issue the Tehcir Law (Law on Deportation), which authorised the deportation of Armenians from the Empire's eastern provinces to Syria between 1915 and 1918. The Armenians were intentionally marched to death and a number were attacked by Ottoman brigands. While the exact number of deaths is unknown, the International Association of Genocide Scholars estimates around 1.5 million. The government of Turkey continues to deny the genocide to the present day, arguing that those who died were victims of inter-ethnic fighting, famine, or disease during World War I; these claims are rejected by most historians.

Other ethnic groups were similarly attacked by the Ottoman Empire during this period, including Assyrians and Greeks, and some scholars consider those events to be part of the same policy of extermination. At least 250,000 Assyrian Christians, about half of the population, and 350,000–750,000 Anatolian and Pontic Greeks were killed between 1915 and 1922.

About 8 million soldiers surrendered and were held in POW camps during the war. All nations pledged to follow the Hague Conventions on fair treatment of prisoners of war, and the survival rate for POWs was generally much higher than that of combatants at the front.

Around 25–31% of Russian losses (as a proportion of those captured, wounded, or killed) were to prisoner status; for Austria-Hungary 32%; for Italy 26%; for France 12%; for Germany 9%; for Britain 7%. Prisoners from the Allied armies totalled about 1.4 million (not including Russia, which lost 2.5–3.5 million soldiers as prisoners). From the Central Powers, about 3.3 million soldiers became prisoners; most of them surrendered to Russians.