Fort Union was established in 1851 by Lieutenant Colonel Edwin V. Sumner as a guardian and protector of the Santa Fe Trail. During it's forty-year history, three different forts were constructed close together. The third and final Fort Union was the largest in the American Southwest, and functioned as a military garrison, territorial arsenal, and military supply depot for the southwest. Today, visitors use a self-guided tour path to visit the second fort and the large, impressive ruins of the third Fort Union. The largest visible network of Santa Fe Trail ruts can be seen here.