The Mexican–American War was an armed conflict between the United States and Mexico from 1846 to 1848. It followed the 1845 U.S. annexation of Texas, which Mexico considered Mexican territory because it did not recognize the Velasco 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 Anglo-American citizens who had moved from the United States to Texas after 1822^[58] wanted to be annexed by the United States.^[59]

Domestic sectional politics in the U.S. were preventing annexation since Texas would have been a slave state, upsetting the balance of power between Northern free states and Southern slave states.^[60] In the 1844 United States presidential election, Democrat James K. Polk was elected on a platform of expanding U.S. territory in Oregon and Texas. Polk advocated expansion by either peaceful means or armed force, with the 1845 annexation of Texas furthering that goal^[61] by peaceful means. However, the boundary between Texas and Mexico was disputed, with the Republic of Texas and the U.S. asserting it to be the Rio Grande and Mexico claiming it to be the more-northern Nueces River. Polk sent a diplomatic mission to Mexico in an attempt to buy the disputed territory, together with California and everything in-between for $25 million (equal to $785,178,571 today), an offer the Mexican government refused.^[62] Polk then sent a group of 80 soldiers across the disputed territory to the Rio Grande, ignoring Mexican demands to withdraw.^[63] Mexican forces interpreted this as an attack and repelled the U.S. forces on April 25, 1846,^[64] a move which Polk used to convince the Congress of the United States to declare war.^[63]