Fort Union stands near the junction of the Mountain and Cimarron branches of the Santa Fe Trail in a landscape long used by Plains and Pueblo peoples. The United States Army established the post in 1851, only a few years after the Mexican-American War transferred New Mexico to United States control. Its mission combined defense of the trail, supply of distant garrisons, enforcement of federal authority, and military campaigns that dispossessed Native nations. The first fort was an irregular cluster of log and earthen buildings hurriedly built along the valley.

Concern about invasion during the Civil War led soldiers to build a second installation in 1861: a large star-shaped earthwork designed for artillery defense. Confederate forces advanced up the Rio Grande in 1862 but were turned back at Glorieta Pass before reaching Fort Union. The earthwork remained strategically significant but proved unhealthy and poorly suited to everyday operations. Its low, grassed outlines survive as archaeological earthworks rather than a restored fort, making the second Fort Union visually distinct from the adobe ruins most visitors associate with the national monument.

Construction of a third fort began in 1863 and produced an expansive territorial-style complex of adobe walls on stone foundations. Officers' quarters, enlisted barracks, storehouses, corrals, shops, hospital facilities, and administrative buildings surrounded organized yards. Beside it, the Fort Union Depot became the quartermaster hub for posts across the Southwest. Freight wagons arriving over the Santa Fe Trail unloaded food, clothing, weapons, building materials, and mail. Civilian employees, Hispanic laborers, soldiers, laundresses, traders, freighters, and families made the post a diverse community as well as a military machine.

Fort Union supported campaigns against Apache, Comanche, Kiowa, Navajo, Ute, and other Native peoples whose resistance challenged invasion of their homelands. Treaties, military force, destruction of resources, and reservation policies must be understood alongside stories of trail protection. After the railroad reached northern New Mexico in 1879, wagon freight declined and the supply system changed. The Army abandoned Fort Union in 1891. Roofs collapsed, reusable materials disappeared, and exposed adobe walls eroded. Local residents and preservationists later advocated protection, leading Congress to establish the national monument in 1954.

Today the National Park Service stabilizes the third fort's original stone foundations and surviving adobe wall cores beneath protective earthen caps. These roofless ruins have not been rebuilt into complete rooms; selective repairs and sacrificial coatings slow erosion while leaving loss visible. Remnants of the first fort, the second fort's earthwork, wagon ruts, and the wider cultural landscape broaden the story beyond the picturesque adobe complex. A visitor center and walking trail provide access under posted conditions. Fort Union records commerce and community, but also the infrastructure through which the United States consolidated colonial power in the Southwest.