Second Fort Union
Situated in the valley east of the first Fort Union, the massive earthen fortification began to take shape in July 1861. Earth parapets formed a square with angles shaped like arrowheads shooting out 200 feet from each corner. Located in these star points were storehouses, company barracks, and officers' quarters. The parapets supported firing platforms and artillery emplacements. Four other angles served as curtains against enemy fire. The structure resembled an eight-pointed star.
An officer proclaimed that it was "as fine a work of its kind as I ever saw" and stated that "all Texas can't take it." Another conclusion was reached after a cannon was fired from atop the mesa to the west. He confirmed that "the work has a dip toward these hills which causes its whole interior to be revealed." This meant that not only could enemy artillery reach the fort, but their mounted guns could not reach the mesa, where attackers would most likely place their cannon. In addition, the star fort did not offer improved living conditions. Like the first fort, the quarters and storehouses were made of unbarked pine logs that quickly rotted and housed nesting insects. Underground rooms were damp and unventilated. With rain the dirt floors quickly turned to mud. Consequently, most of the soldiers camped in tents surrounding the fort. Work continued on the second fort until June 1862. By then New Mexico's fate had been decided. There was no longer any need for such a fortification.
Brig. Gen. Henry H. Sibley's brigade marched north from Fort Bliss in January 1862, aiming for Albuquerque, Santa Fe, Fort Union, and eventually Denver. By March 5 the Confederate Army had taken the capital of Santa Fe. Only Fort Union stood between them and Denver. Soldiers from Fort Union joined forces with New Mexico and Colorado Volunteers, and headed south to meet the Confederate Army at Glorieta.
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. With the loss of Sibley's supply train went the "grand design for the Confederacy in the West." The Battle of Glorieta Pass had successfully eliminated the Confederate threat in the western territories.
National Park Service
Civil War-era earthen fort at Fort Union National Monument, New Mexico
"Although the Second Fort’s general configuration is discernible from its current surface topography, the precise nature of its mid-nineteenth-century construction and layout presently remains unknown. It encompasses 33-acre site, and although a map of the Second Fort currently exists, partially derived from the fort’s present-day surface topography, that map is largely conjectural, based upon assumptions about how the fort ought to have been configured, presuming it had been laid out in strict accord to established, mid-nineteenth-century U.S. Army protocols. The actual fort likely deviates from such idealized military specifications, yet exactly how and to what extent it deviates is presently unknown.
Fort Union's second fort, a massive, bastioned earthwork constructed in 1861 and largely abandoned by the close of 1863, stands as a unique and important example of mid-nineteenth-century American military architecture. The fort is exceptional for several reasons: Firstly, it is integrally associated with the Battle of Glorieta Pass (March 26-28, 1862), a crucial western engagement of the Civil War and a decisive Union victory that ended Confederate incursions into the American Southwest. Secondly, it is the sole surviving earthen star fort erected west of the Mississippi River. And thirdly, it is the most intact, least-disturbed Civil War-era bastioned earthen fort surviving anywhere within the United States today. For all of these important reasons the Second Fort is exemplary and warrants closer, more methodical examination than it has received to date.
A proposed multi-phased, interdisciplinary field study of the Second Fort (one involving geophysicists, archeologists, historical architects, military historians, and others) may yield valuable new insights into Civil War-era earthen fort construction and use. Concomitantly, the NPS’s interpretations of such earthen fortifications will improve—not only at Fort Union National Monument, but at other, more easterly NPS units also containing Civil War-era earthen forts (e.g., Fort Donelson NB in Tennessee; Fort Foote Park in Maryland).
The Civil War-era earthen fort at Fort Union National Monument was added to the National Register of Historic Places in October 1966."