Mexican–American War

The Mexican–American War was fought between the United States and Mexico from 1846 to 1848. It grew out of the American annexation of Texas and a dispute over the border, and ended with the US occupation of Mexico City and the Treaty of Guadalupe Hidalgo, by which Mexico gave up roughly half of its territory. In Mexico it is known as the United States intervention in Mexico.

Texas had broken away from Mexico in 1836, when rebels defeated the forces of President Antonio López de Santa Anna. Mexico never recognised the independence of the Republic of Texas. The question of annexing Texas to the United States was bound up with slavery, since Texas would enter as a slave state, but in 1845, after the expansionist Democrat James K. Polk won the presidency, Congress voted to admit it. The two countries also disagreed about the border: Texas claimed the Rio Grande, while Mexico held that the province had always ended at the Nueces River, further north.

Polk also wanted California and the lands between. He sent the diplomat John Slidell to offer to buy them, but the Mexican government refused to receive him.

In early 1846 Polk ordered General Zachary Taylor to advance to the Rio Grande. On 25 April 1846 Mexican cavalry attacked an American patrol in the disputed zone, an event known as the Thornton Affair. Polk told Congress that Mexico had shed "American blood upon American soil", and Congress declared war on 13 May. Some Americans opposed the war as an aggressive grab for land and slave territory; among them was a young congressman, Abraham Lincoln, who challenged Polk to identify the exact spot where blood had been shed.

Taylor defeated Mexican forces at Palo Alto and Resaca de la Palma in May 1846, captured Monterrey in September, and in February 1847 repelled a much larger army under Santa Anna at Buena Vista. Meanwhile, General Stephen Kearny occupied Santa Fe and New Mexico with little resistance, and American settlers, naval forces and soldiers took control of California, where fighting with Californio forces continued until January 1847.

As Mexico still refused to negotiate, Polk sent General Winfield Scott to strike at the capital. Scott's army landed near Veracruz in March 1847 and took the port after a siege. Moving inland, it defeated Santa Anna at Cerro Gordo in April and, after a series of battles around the capital in August and September, stormed the castle of Chapultepec on 13 September. Mexico City was occupied the next day. Mexico honours the young military cadets who died defending Chapultepec as the Niños Héroes. Several future generals of the American Civil War, including Ulysses S. Grant and Robert E. Lee, served in the war.

The Treaty of Guadalupe Hidalgo was signed on 2 February 1848. Mexico accepted the Rio Grande border and ceded California, Nevada, Utah, most of Arizona and New Mexico, and parts of Colorado and other states, together with its claim to Texas. In return the United States paid 15 million dollars and took over about 3.25 million dollars in debts owed to American citizens. The Gadsden Purchase of 1853 added a further strip of land in the south-west.

The war cost the United States about 13,000 dead, the great majority from disease rather than combat. Mexican losses are less certain; one estimate puts Mexican deaths, including civilians and deaths from disease, at around 25,000. For Mexico, defeat was a national trauma that deepened political instability. For the United States, the new lands, where gold was discovered in California in 1848, made it a continental power, but the dispute over whether slavery should spread to them sharpened the sectional conflict that led to civil war in 1861.

Our 1815 map shows New Spain on the eve of Mexican independence, and the 1878 map the border between Mexico and the United States as the treaty and the Gadsden Purchase left it.