February 2, 1848
Mexico won its independence from the Spanish Empire back in 1821 during the Mexican War of Independence. In 1845 (during his last weeks in office), President John Tyler signed legislation that authorized the annexation of the Republic of Texas, a move that was viewed by the Mexican government as an act of war. Mexico never viewed the Republic of Texas as an independent country. The United Kingdom and France both tried to persuade Mexico to stay out of war with the United States, but to no avail. In April of 1846, Mexican forces attacked Americans near the Rio Grande (in close proximity to future President Zachary Taylor's camp) in a raid known as the Thornton Affair. Within a month, Congress passed (and President James Polk signed) a declaration of war. By July, Mexican Congress had done the same.