The Mexican-American War, fought between 1846 and 1848, was a defining moment in the history of the United States and Mexico. It marked the first time the United States engaged in a war outside its borders and resulted in the acquisition of a large swath of land that would become the American Southwest. The war was sparked by a dispute over Texas, which had been annexed by the United States in 1845, and a desire by President James K. Polk to acquire more territory in the West.