The Mexican–American War,[a] also known in the United States as the Mexican War and in Mexico as the American intervention in Mexico,[b] was an armed conflict between the United States of America and the United Mexican States (Mexico) from 1846 to 1848. It followed in the wake of the 1845 American annexation of the independent Republic of Texas. The unstable Mexican caudillo leadership of President/General Antonio López de Santa Anna still considered Texas to be its northeastern province and never recognized the Republic of Texas, which had seceded a decade earlier. In 1845, newly elected U.S. President James K. Polk sent troops to the disputed area and a diplomatic mission to Mexico. After Mexican forces attacked American forces, Polk cited this in his request that Congress declare war.
The Wilmot Proviso was designed to eliminate slavery within the land acquired as a result of the Mexican War (1846-48). Soon after the war began, President James K. Polk sought the appropriation of $2 million as part of a bill to negotiate the terms of a treaty. Fearing the addition of a pro-slave territory, Pennsylvania Congressman David Wilmot proposed his amendment to the bill. Although the measure was blocked in the southern-dominated Senate, it enflamed the growing controversy over slavery, and its underlying principle helped bring about the formation of the Republican Party in 1854.
The antislavery declaration reflected the national political situation. The Democrats had divided over slavery and expansion during the 1844 election, but after his victory James K. Polk had pushed for the acquisition of the Oregon country and for a larger share of Texas from Mexico.