Fort Union in northeastern New Mexico grew at a key junction of the Santa Fe Trail after the United States reorganized defenses in territory gained in 1848. Established in 1851, it served first as a frontier army post and support station for travelers, then expanded through three successive forts into the territory’s largest military installation. Its troops fought in campaigns across the Southwest and contributed to the Union victory at Glorieta Pass in 1862. As railroads replaced trail traffic, the fort’s supply role waned, and it was abandoned in 1891. The preserved remains also reflect a long dispute over the Mora Land Grant on which the army had built.