Mexican-American War The Mexican War was a war that lasted from 1846 to 1848. The two major issues behind the war were the inability of the Mexican government to establish political and economic control over its vast northern frontier, including the Mexican state of Tejas y Coahuila, and the westward movement and dynamic expansionism of the United States during the 19th century. Before the war occurred, Mexico had already lost control of much of its northeastern territory as a result of the Texas Revolution. This land, combined with the territory of Mexico at the end of the war, would eventually form what is now part of the United States to