A bustling center of frontier defense in the Southwest for four decades, Fort Union was the largest U.S. military post in the region and a base for both military and civilian ventures that molded its destiny. Astride the southern end of the Mountain Branch of the Santa Fe Trail near the point where it merged with the southern terminus of the Cimarron Cutoff, the post was one of the most important of a string established in New Mexico and southern Arizona in the area acquired from Mexico in the Mexican War (1846-48).

The fort's mission was broad. It protected the Santa Fe Trail, on which it was a resting place and refitting point and a stopping point for Independence, Missouri-Santa Fe mail coaches; was the major Army supply depot in the Southwest; served as a transportation center for civilian wagon freighters carrying military supplies; and provided a base for campaigns that penetrated the homeland of the Apache, Ute, Navajo, Kiowa, and Comanche. Also, in a phase of history extraneous to this volume, the fort played a strong role in repelling the Confederate invasion of New Mexico from Texas in 1862; it was the prime staging area and logistical base and supported the force of Colorado Volunteers that won a victory over the southerners at the Battle of Glorieta Pass, New Mexico in March, 1862.

Three different Fort Unions existed over the years. The first, a shabby collection of log buildings, was erected in 1851, only five years after the U.S. conquest of New Mexico, on the west bank of Coyote Creek. The second post, begun in 1861 in preparation for the Confederate thrust from the south, was located across the creek from the first. A massive earthwork fort in a star shape, it had ditches, parapets, and bombproofs. Work continued intermittently until June 1862, by which time the need for the fortification had passed. The last fort, a large complex of adobe structures in the Territorial architectural style, was begun in 1863 and finished in 1869. It was situated in the same area as the star fort, except for the arsenal, built on the site of the first fort.

Probably the most dramatic duty of the garrison, particularly at the time of Indian uprisings, was furnishing escorts and other protection for the Santa Fe Trail. Dragoons and Mounted Riflemen focused their efforts on the Cimarron Cutoff, which extended northeastward to the Cimarron Crossing of the Arkansas River.