Max G's Mexican American War Timeline By MsOlson Period: Dec 1, 1845 to Mar 1, 1848 Main Dec 6, 1845 US Tried to Buy Land (Slidell and Polk) President Polk sends John Slidell to Mexico to attempt to purchase California and New Mexico from Mexican President Herrera. Dec 29, 1845 Texas Admitted to the Union Texas joined the Union in 1845, with Mexico breaking diplomatic relations with the Union and later fighting multiple battles in the new state of Texas. Jan 15, 1846 US Troop Stationed at Diputed Border President Polk moves troops to the Rio Grande to prepare for war. May 13, 1846 US Declares War After a short debate Congress declares war on Mexico after they cross the US border and shed blood on US soil. Sep 21, 1846 Battle of Monterrey US Army defeats Mexican Army of the North, pushing the Mexican army farther south. Sep 14, 1847 Mexico City Surrendered Mexico finally succumbs to US forces and surrenders the Mexican capital, Mexico City Feb 2, 1848 Treaty of Guadalupe Hidalgo Mexico and the US sign the Treaty of Gualdalupe Hidalgo, establishing the new border and other negotiations.