helenh.'s mexican american war timeline By MsOlson Period: Jan 6, 1845 to Jan 6, 1849 helenh.'s mexican american war timeline Mar 1, 1845 President Tyler signs congressional annex for texas texas joins the union. Nov 1, 1845 US tries to buy land (Sidel and Polk) Mar 1, 1846 US troops march into disbuted territory. May 1, 1846 US declares war. May 8, 1846 Battle of Palo Alto First major battle. Sep 25, 1846 Battle of Monterrey US victory. Sep 8, 1847 Battle of Molino del Rey the US was outneumbered but still won Sep 12, 1847 Battle of Mexico City Mexico forfeited to the US Feb 2, 1848 treaty of Guadelupe Hidalgo US won from a Mexican forfeight.