1846 – Mexican–American War: The United States declares war on the Federal Republic of Mexico following a dispute over the American annexation of the Republic of Texas and a Mexican military incursion. On May 13, 1846, the United States Congress passed An Act providing for the Prosecution of the existing War between the United States and the Republic of Mexico, thereby declaring war against Mexico. The declaration resulted in the Mexican–American War (1846–48). The act laid out regulations for the size and organization of the militia to participate in the war, how they were to be recruited, and the amount of money appropriated for the war—10 million dollars. The Mexican–American War, also known in the United States as the Mexican War and in Mexico as the United States intervention in Mexico,[c] (April 25, 1846 – February 2, 1848) was an invasion of Mexico by the United States. It followed the 1845 American annexation of Texas, which Mexico still considered its territory because it refused to recognize the Treaties of Velasco, signed by President Antonio López de Santa Anna after he was captured by 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 wanted to be annexed by the United States. Sectional politics over slavery in the United States had previously prevented annexation because Texas would have been admitted as a slave state, upsetting the balance of power between Northern free states and Southern slave states. In the 1844 United States presidential election, Democrat James K. Polk was elected on a platform of expanding U.S. territory to Oregon, California (also Mexican territory), and Texas by any means, with the 1845 annexation of Texas furthering that goal. 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 northerly 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 (equivalent to $798 million in 2024), an offer the Mexican government refused. Polk then sent a group of 80 soldiers across the disputed territory to the Rio Grande, ignoring Mexican demands to withdraw. Mexican forces interpreted this as an attack and repelled the U.S. forces on April 25, 1846, a move that Polk used to convince the Congress of the United States to declare war. The 1848 Treaty of Guadalupe Hidalgo ended the war, and Mexico recognized the cession of present-day Texas, California, Nevada, and Utah as well as parts of present-day Arizona, Colorado, New Mexico, and Wyoming. The U.S. agreed to pay $15 million (equivalent to $479 million in 2024) for the physical damage of the war and assumed $3.25 million of debt already owed by the Mexican government to U.S. citizens. Mexico relinquished its claims on Texas and accepted the Rio Grande as its northern border with the United States.
