The
Treaty of Guadalupe Hidalgo, which ended the
Mexican-American War in February 1848, gave the U.S. undisputed control of Texas and ceded to the United States the present-day states of California, Nevada, Utah, most of New Mexico, Arizona, and Colorado, and parts of Texas, Oklahoma, Kansas, and Wyoming. In New Mexico, the U.S. army set up garrisons in settlements to protect the area's inhabitants and travel routes from raids by Native Americans, but this proved unsatisfactory. Temptations in civilian communities such as alcohol distracted soldiers and often made them unfit for duty. In April 1851, Lt. Col.
Edwin V. Sumner was ordered to revise the defense of the territory. Among his first acts was to break up the garrisons and move them closer to the Indians. He moved his headquarters and supply depot from Santa Fe to near where the Mountain and Cimarron branches of the Santa Fe Trail converged. This was about 25 miles northeast of the town of Las Vegas. There he set up Fort Union. The first of the three forts built in this valley was begun in August 1851. For a decade it served as the base for military operations in the area and a key station on the Santa Fe Trail, affording travelers a place to rest nearby and refit at the post sutler's store (where general merchandise not supplied to soldiers by the army was sold). It also became the principal military supply depot of the Southwest. During the 1850s, the fort's mounted riflemen (called dragoons) campaigned against several southern Rocky Mountain Indian tribes that were disrupting traffic on the Santa Fe Trail. One of the first campaigns was directed against the Jicarilla Apaches. In 1854, that tribe nearly wiped out a company of dragoons. The Apaches were driven into the mountains west of the Rio Grande and routed. Military operations were also conducted against Utes of southern Colorado in 1855 and Kiowas and Comanches for raiding the plains east of the fort in 1860–61. Anticipating a Confederate invasion of New Mexico, Col.
Edward R.S. Canby, charged with the territory's defense, concentrated troops at
Fort Craig on the Rio Grande near present-day Socorro, and sent its soldiers to patrol the Santa Fe Trail, the main artery of supply for federal forces. He also ordered construction of the second Fort Union, a star-shaped earthen fortification, to strengthen defenses. When the Civil War began in April 1861, most of the regular troops (except those officers who joined Confederate forces) were withdrawn from Fort Union and other frontier posts to be sent east. They were replaced by volunteer regiments. The second fort at the national monument was designed to defend against a Confederate military invasion coming north up the Rio Grande Valley, from
El Paso, Texas. Early in the war, such a force was turned back in 1862 by Colorado and New Mexico volunteers and U.S. regulars from Fort Union. They fought the Confederates at
Glorieta Pass, about 20 miles southeast of Santa Fe. Defeated, the Confederates withdrew to Texas, ending Civil War activity in the Southwest and saving the mines in Colorado from being used as a source of funds for the South. The second Fort Union was abandoned soon afterwards. Construction of the third fort began in 1863. With New Mexico securely in federal hands, the new departmental commander, Brig. Gen. James H. Carleton began its construction. The sprawling installation took six years to complete and was the most extensive in the territory. It included not only a military post, but also a separate quartermaster depot with warehouses, corrals, shops, offices, and living quarters. The supply function of the fort overshadowed that of the military and employed far more men, most of which were civilians. An ordnance depot, erected on the site of the first fort at the western edge of the valley, rounded out the complex. Throughout the 1860s and the '70s, troops from Fort Union took part in operations against the Native Americans. Several relentless campaigns against the Apaches, Navajos, Cheyennes, Arapahos, Kiowas, Utes, and Comanches finally brought peace to the Southern Plains in the spring of 1875 on the government's terms. Fort Union's involvement in the Indian wars had come to an end, but its garrison occasionally helped to track down outlaws, quell mob violence, and mediate feuds. The supply depot continued to flourish until 1879, when the Santa Fe Railroad replaced the Santa Fe Trail as the principal means of commerce. By 1891, the fort had outlived its usefulness and was abandoned. == Units assigned to Fort Union and Kit Carson ==