On May 13, 1846, the United States Congress declared war upon Mexico. Although the Mexican-American War lasted only 18 month, its consequences were profound. Mexico lost nearly one-half its territory; the United States acquired most or all the future states of Texas, California, New Mexico, Arizona, Nevada, Utah, and Colorado. During the Mexican-American War, the United States military made its first successful offensive on foreign soil, capturing Mexico City - the enemy capital. Some historians have described this offensive as America's first step toward empire. More ominously, the Mexican-American War raised the explosive issue of whether slavery should be permitted in the newly acquired territories. Less than two decades later, this issue would help to speed America toward its greatest tragedy: The Civil War.