Background: Fort Union was built shortly after the Mexican American War of the 1840's, to build an American presence in the new acquired area of New Mexico. The fort was used as a central depot for shipping supplies to other forts in the region. In addition, the troops were involved in fighting Indians and served as a Union Army outpost during the Civil War. The remained in active use until the expansion of railroads to Santa Fe, NM, and other parts of the Southwest made it obsolete. The fort was decommissioned in 1891, and is now a national monument.