Colonel Edwin Vose Sumner established Fort Fillmore to guard a critical chokepoint where several major emigrant routes converged. The Upper and Lower Emigrant Trails met at El Paso before continuing west through Apache territory; the Butterfield Overland Mail would later use the same corridor. The fort was originally built in the jacal style -- upright wooden posts covered with adobe plaster -- though more substantial walls eventually replaced the crude first construction. Mexican laborers made the adobe bricks while soldiers did most of the building work. The location proved problematic from the start: the Rio Grande later shifted its course, leaving the fort a mile from reliable water and forcing the Army to haul supplies by wagon. This made the post difficult to defend and expensive to maintain.

The roster of soldiers who served at Fort Fillmore reads like a preview of Civil War fame. Captain George Pickett, who would achieve immortality leading the doomed charge at Gettysburg on July 3, 1863, was stationed here in the 1850s. General Ambrose Burnside used the fort as a supply base in 1855 while drilling geothermal wells fifteen miles to the west. The 1st and 2nd Dragoons rotated through, along with the Regiment of Mounted Rifles and several infantry regiments. These units participated in the Gila Expedition of 1857 and numerous operations against the Apaches in the Sacramento Mountains. Captain Henry W. Stanton was killed near the Rio Penasco during one such foray; Fort Stanton would later bear his name. His grave was one of the few that could still be identified when inspectors examined the abandoned post in 1869.

When Confederate Lieutenant Colonel John Baylor rode into Mesilla on July 25, 1861, and repulsed a Union attack, the fate of Fort Fillmore was sealed. Major Isaac Lynde commanded the garrison, and after his failed assault on Mesilla, he found himself trapped. Confederate forces captured most of the fort's horses that night, eliminating any chance of rapid movement. Fearing a siege he could not withstand, Lynde ordered the fort abandoned on July 27. His men destroyed what supplies they could and set fire to the citadel before beginning their retreat northeast toward Fort Stanton, eighty miles away across the Organ Mountains. The march became a disaster. Soldiers who had filled their canteens with medicinal whiskey instead of water collapsed in the desert heat. Confederate cavalry caught up with the straggling column at San Augustine Springs, where Lynde surrendered his entire command -- nearly 500 men -- to a smaller Confederate force.

The Confederates occupied Fort Fillmore briefly before Union forces returned to the region in 1862. On August 7 of that year, Federal troops near the fort skirmished with Confederate soldiers retreating from Santa Fe and defeated them. The fort was officially closed in October 1862, though it continued to appear on maps and travelers' accounts as a waypoint along the western trails. The buildings slowly deteriorated. At some point -- the exact date is lost -- the remaining structures were leveled after a failed attempt to sell or trade the property to New Mexico as a park. Today, a grove of pecan trees grows where the parade ground once stood, and the fort's cemetery remains on a sandy ridge to the southeast, its dead still buried in unmarked graves more than 150 years after the last soldiers left.