of the Civil War in 1861, Tappan received a commission from Colorado's Territorial Governor William Gilpin to help raise a regiment of Union volunteer troops. Commissioned initially as a captain, his success in recruitment drives in the small Colorado mining towns of Black Hawk, Georgetown, Golden, and Central City resulted in his being promoted by Gilpin to Lieutenant Colonel of the newly formed First Colorado Volunteer Regiment, serving under Colonel John P. Slough, a lawyer from Cincinnati, Ohio. After helping train the regiment at Camp Weld near Denver, Tappan was placed in command of Fort Wise with a detachment of the regiment until news arrived of the invasion of New Mexico Territory by Confederates from Texas. Service in New Mexico Territory Tappan and his charges joined the hurried movement of the First Colorado Volunteers to reinforce Fort Union in New Mexico in early March 1862, and later saw action against the Confederate troops along the Santa Fe Trail in New Mexico. Tappan participated in the second day of the Battle of Glorieta Pass on March 28, 1862, at Pigeon's Ranch. Tappan was the effective field commander of the Federal forces during the main engagement that day, and was exposed to enemy fire while Colonel Slough directed the Federal activity from the rear. Tappan's actions that day were eventually overshadowed by the later success of the one-third of Slough's command that bypassed the Confederate lines and attacked and destroyed the enemy wagon train and supplies in their rear near Johnson's Ranch. Commanded by Major John M. Chivington, a former Methodist minister, who had also commanded the advance Union force which tangled with and defeated an advance Confederate unit on March 27 at Apache Canyon, this Union detachment effectively ended the threat posed by the Confederate invaders by destroying their supplies and ammunition stores. When Slough resigned as colonel a few days after the victory at Glorieta Pass, Tappan was the ranking officer and acting colonel. Although later recognized by both Col. Edward R. S. Canby, Commander of the Department of New Mexico, and Col. Christopher "Kit" Carson of the First New Mexico Volunteers for his sound military abilities, Tappan voluntarily relinquished his seniority rights and joined in signing a petition from among the men of the First Colorado to elevate Chivington, the recent hero of the Glorieta battle, to colonel. Chivington's promotion to Colonel where he outranked Tappan would eventually produce