The Mexican–American War, also known as the Mexican War, the U.S.–Mexican War or the Invasion of Mexico, was an armed conflict between the United States of America and the United Mexican States from 1846 to 1848. It followed in the wake of the 1845 US annexation of Texas, which Mexico considered part of its territory, despite the 1836 Texas Revolution.
After its independence in 1821 and brief experiment with monarchy, Mexico became a republic in 1824, characterized by considerable instability, so that when war broke out in 1846, Mexico was ill-prepared for this conflict. The war with the United States followed in the wake of decades of American Indian raids in the sparsely settled north of Mexico, which prompted the Mexican government to sponsor American migration to the Mexican province of Texas to act as a buffer. Americans and some Mexicans revolted against the Mexican government in the 1836 Texas Revolution, creating a republic not recognized by Mexico, which still claimed it as its national territory. The 1845 expansion of US territory with its annexation of Texas escalated the dispute between the United States and Mexico to open war.

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