The Colorado Territory was formally created in 1861 shortly before the bombardment of Fort Sumter sparked the American Civil War. Although sentiments were somewhat divided in the early days of the war, Colorado was only marginally a pro-Union territory (four statehood attempts were thwarted, largely by Confederate sympathizers in July 1862, February 1863, February 1864, and January 1866).
Colorado was strategically important to both the Union and Confederacy because of the gold and silver mines there as both sides wanted to use the mineral wealth to help finance the war. The New Mexico Campaign (February to April 1862) was a military operation conducted by Confederate Brigadier General Henry Sibley to gain control of the Southwest, including the gold fields of Colorado, the mineral-rich territory of Nevada and the ports of California.
The campaign was intended as a prelude to an invasion of the Colorado Territory and an attempt to cut the supply lines between California and the rest of the Union. However, the Confederates were defeated at the Battle of Glorieta Pass in New Mexico and were forced to retreat back to Texas, effectively ending the New Mexico Campaign.