The deep ruts in the earth record Fort Union's role as guardian of the Santa Fe Trail, a network of routes used for hundreds of years by American Indians, Spaniards, Mexicans, New Mexicans, and Americans that brought economic success to towns along its path and, with the presence of a growing number of Americans, promoted westward expansion for the United States. Established in 1851 to protect this vital international trade route, Fort Union ensured that commerce traveling over 900 miles from the United States (Missouri) to Mexico (Santa Fe) reached its destination safely. The fort also used the Santa Fe Trail as its primary military supply route, bringing supplies and men to the frontier as conflicts grew with American Indian populations. After the railroad arrived in 1879, the Santa Fe Trail instantly became obsolete, eliminating the need for Fort Union, which was abandoned in 1891.