Mexican War

War between the USA and Mexico 1846–48. The war was ostensibly over disputed boundaries between the two nations, but it was also an excuse for the USA to pursue its ‘manifest destiny’ to expand westwards. US forces defeated the Mexican army in a series of battles in the disputed regions and Mexico itself, and occupied Mexico City in 1847. Under the Treaty of Guadaloupe Hidalgo that ended the war, the USA acquired what are now California, Nevada, and Utah, and parts of New Mexico, Arizona, Colorado, and Wyoming, all in exchange for $15 million.

Tensions were high between the USA and Mexico as a result of continuing border disputes and the annexation of Texas in 1845 (see Texas, annexation of). President James Polk determined to pursue his notion of manifest destiny for the USA and dispatched General Zachary Taylor to add the disputed territories, by force if necessary. After repeated defeats and invasion of its home territory, a Mexican government was formed that was willing to negotiate a settlement. Presidential envoy Nicholas Trist was ordered home, but he ignored his orders and negotiated the pact ceding vast Mexican territories to the USA. Polk was enraged but had little choice but to submit the exceptionally favourable treaty to the Senate, which ratified it.

Prequel to the war Following the annexation of Texas, President Polk sent US minister John Slidell to Mexico to reach an agreement whereby Mexico would accept the boundary of Texas as the Rio Grande and sell New Mexico and California to the USA for $25 million. The Mexican government refused. Meanwhile, Polk sent General Taylor with an army of occupation into the disputed Texas territory, and ordered them to advance to the Rio Grande. On 25 April, just north of the river, a small Mexican force attacked and defeated US soldiers. On 8 and 9 May 1846, Taylor and his soldiers defeated Mexican army units at Palo Alto and Resaca de la Palma, enabling him to cross the Rio Grande. On 13 May 1846 US Congress formally declared war on Mexico.

Outcome US forces took control of New Mexico in 1846 and California in 1847. In Mexico US forces defeated the Mexican army, led by President Antonio López de Santa Anna, in a series of battles culminating in the occupation of Mexico City in May 1847. Despite repeated defeats and invasion of his country, Santa Anna refused to negotiate a peace treaty with the USA. US envoy Nicholas Trist, who had been sent by Polk to negotiate the agreement, was ordered home, but Santa Anna resigned and Mexico formed a new government willing to accept a treaty. Trist ignored his orders to return home and negotiated a pact that ceded more than 1,360,000 sq km/525,000 sq mi of Mexican territory to the USA for the sum of $15 million. Although Polk was furious he was forced to submit Treaty of Guadaloupe Hidalgo to the Senate, which ratified it on 2 February 1848.

The acquisition of new territories focused the USA on the issue of slavery. California was admitted to the union in 1850 under a constitution banning slavery, but under the Compromise of 1850 the other territories were organized without any regulation in respect of slavery, leaving it up to their settlers to vote on whether or not there should be ownership of slaves.