Fort Union National Monument, site of three successive forts built (1851, 1861, 1863–68) by the U.S. Army near Watrous in northern New Mexico, about 60 miles (95 km) northeast of Santa Fe. The fort, at a junction of two branches of the Santa Fe Trail, protected settlers on the trail and was an important supply depot. The first fort was established by Lieutenant Colonel Edwin V. Sumner in 1851. An earthen fort was built nearby in 1861 to hold back an invasion by Confederate troops during the American Civil War. The third version of the fort took five years to complete and was the largest in the Southwest. It was abandoned in 1891 and fell into ruin.
The 720-acre (291-hectare) site was designated a national monument in 1954. There is a self-guided trail through the ruins, and the visitors’ centre displays historical exhibits. Wagon wheel ruts on the Santa Fe trail are still visible.