The Mexican–American War (25 April 1846 – 2 February 18486), also known in the United States as the Mexican war was invasion of Mexico by the United States Army. It followed the 1845 American annexation of Texas, which Mexico still considered its territory because it refused to recognize the Treaties of Velasco. The Republic of Texas was de facto an independent country, but most of its Ango-American citizens who had moved from the United States to Texas after 1822 wanted to be annexed by the United States.

     Texas gained its independence from Mexico in 1836. Initially, the United States declined to incorporate it into the union, largely because northern political interests were against the addition of a new state that supported slavery. The Mexican government was also encouraging border raids and warning that any attempt at annexation would lead to war. Nonetheless, annexation procedures were quickly initiated after the 1844 election of Polk, a firm believer in the doctrine of Manifest Destiny, who campaigned that Texas should be “re-annexed.” When his offer to purchase those lands was rejected, he instigated a fight by moving troops into a disputed zone between the Rio Grande and Nueces River that both countries had previously recognized as part of the Mexican state of Coahuila.

     Finally, on 2 February 1848, the Treat of Guadalupe Hidalgo was signed, establishing the Rio Grande (and not the Nueces River) as the U.S.-Mexican border. Under the treaty, Mexico also recognized the U.S. annexation of Texas, and agreed to sell California and the rest of its territory north of the Rio Grande for $15 million plus the assumption of certain damage claims.

Source: History.com and Wikipedia (23 August 2025)