- President Polk desired to expand U.S. territory to include California but Mexico refused American offers to purchase the land, leading to bitter relations between the two countries. - Tensions further increased due to a boundary dispute between Texas and Mexico and Mexico's refusal to pay debts owed to American citizens, culminating in clashes between Mexican and American troops in 1846 that Polk used to gain approval for war from Congress. - The U.S. prevailed in the war, gaining control of Texas, California, and much of the present-day American Southwest through the 1848 Treaty of Guadalupe Hidalgo, which ended the war.