The second Fort Union was fundamentally different from the first and third Fort Unions. The second Fort Union was built as a traditional defensive structure, designed to withstand a seige. The first and third Fort Unions were built as frontier forts, designed more like open campuses as places to house and feed soldiers. The army did not anticipate major assaults on its frontier forts.
The second Fort Union was built very quickly by newly recruited soldiers working day and night to finish the massive earthen fort in less than a year. An impending Confederate invasion of New Mexico created the urgency for the second fort. The new fort was located about a mile east of the first Fort Union--away from the mesa sheltering the first fort but still near the water supply at Wolf Creek. The mesa top was a potentially devastating location for opposing artillery and would have enabled the Confederates to capture quickly the first fort.
As the second fort was being completed in early 1862, the Confederates were already marching north up the Rio Grande Valley. Under the command of Brig. Gen. Henry H. Sibley, the Confederates were on their way to Albuquerque, Santa Fe, Fort Union and eventually the Colorado gold fields. By early March 1862, the Confederates had taken Santa Fe, with only Fort Union standing between the invaders and the Colorado gold fields.
Soldiers from Fort Union joined forces with Colorado Volunteers and headed south to challenge the Confederate army. The two armies confronted one another at Glorieta Pass, in between Santa Fe and Pecos, NM. The Battle of Glorieta Pass, fought from March 26-28, 1862, was the decisive battle in the New Mexico Campaign during the Civil War in the West. Union forces, under the command of Col. John P. Slough, 1st Colorado Volunteers, and under the direction of Maj. John M. Chivington, 1st Colorado Volunteers, successfully defeated the Confederate forces when the southerners' wagon trains were destroyed. With the loss of their supplies, the Confederates had little choice but to retreat back to Texas. Once it became clear that the Battle of Glorieta Pass had successfully eliminated the Confederate threat in the West, the army began laying plans for the third Fort Union.