The Battle of Glorieta Pass, the decisive battle of the New Mexico Campaign of the American Civil War, was fought near the pass in March 1862. The victory by the Union Army (primarily in the form of the Colorado Militia) prevented the breakout of the Confederate Army forces onto the High Plains on the east side of Sangre de Cristo Mountains, halting the intended Confederate advance northward along the base of the Rocky Mountains. The battle is commemorated at Pecos National Historic Park on the east side of the pass. In the 20th century, the pass became used as the route of U.S. Highway 84 and later Interstate 25. The town of Glorieta is located on the eastern side of the pass.