The Mexican–American War was an armed conflict between the United States of America (US) and Mexico from 1846 to 1848.
It is also known in the US as the Mexican War and in Mexico as the Intervención Estadounidense en México (US intervention in Mexico), US-Mexican War, and US-Mexico War.
It followed the 1845 US annexation of Texas, which Mexico still considered Mexican territory since the government did not recognise the treaty signed by Mexican General Antonio López de Santa Anna when he was a prisoner of the Texian Army during the 1836 Texas Revolution.
The Republic of Texas was de facto an independent country, but most of its citizens wished to be annexed by the US. Domestic sectional politics in the US prevented that since Texas would have been a slave state, upsetting the balance of power between northern free states and southern slave states.
In the 1844, US presidential election, Democrat James K. Polk was elected on a platform of expanding US territory in Oregon and Texas. Polk advocated expansion by either peaceful means or by armed force, with the 1845 annexation of Texas as furthering that goal.
For Mexico, this was itself a provocation, but Polk went further, sending US Army troops to the area; he also sent a diplomatic mission to Mexico to try to negotiate the sale of territory. US troops’ presence was provocative and designed to lure Mexico into starting the conflict, putting the onus on Mexico and allowing Polk to argue to Congress that a declaration of war should be issued.
Mexican forces attacked US forces, and the US Congress declared war.
Beyond the disputed area of Texas, US forces quickly occupied the regional capital of Santa Fe de Nuevo México along the upper Rio Grande, which had trade relations with the US via the Santa Fe Trail between Missouri and New Mexico.
US forces also moved against the province of Alta California, and then moved south.
The Pacific Squadron of the US Navy blockaded the Pacific coast farther south in the lower Baja California Territory. The Mexican government refused to be pressured into signing a peace treaty at this point, making the US invasion of the Mexican heartland under Major General Winfield Scott and its capture of the capital Mexico City a strategy to force peace negotiations.
Although Mexico was defeated on the battlefield, politically its government’s negotiating a treaty remained a fraught issue, with some factions refusing to consider any recognition of its loss of territory.
Although Polk formally relieved his peace envoy, Nicholas Trist, of his post as negotiator, Trist ignored the order and successfully concluded the 1848 Treaty of Guadalupe Hidalgo. It ended the war and Mexico recognised the Mexican Cession, areas not part of disputed Texas but conquered by the US Army. These were northern territories of Alta California and Santa Fe de Nuevo México to the United States.
The US agreed to pay $15 million for the physical damage of the war and assumed $3.25 million of debt already owed earlier by the Mexican government to US citizens. Mexico acknowledged the loss of what became the State of Texas and accepted the Rio Grande as its northern border with the US.
Aftermath and Legacy
The victory and territorial expansion Polk envisioned, inspired patriotism among some sections of the US, but the war and treaty drew fierce criticism for their casualties, monetary cost, and heavy-handedness, particularly early on.
The question of how to treat the new acquisitions also intensified the debate over slavery in the US. Although the Wilmot Proviso that explicitly forbade the extension of slavery into conquered Mexican territory was not adopted by Congress, debates about it heightened sectional tensions. Most scholars see the Mexican-American War as leading to the American Civil War (1861-1865) with many officers trained at West Point playing prominent leadership roles on each side.
In Mexico, the war worsened domestic political turmoil. Since the war was fought on home ground, it suffered a large loss of life of not only its soldiers, but also its civilian population. The nation’s financial foundations were undermined, the territory was lost, and national prestige left it in what prominent Mexicans called a “state of degradation and ruin… [As for] the true origin of the war, it is sufficient to say that the insatiable ambition of the United States, favored by our weakness, caused it.”