Patrick Breen served in 12th Infantry Regiment during the Civil War.
The 2nd Battalion, 12th Infantry Regiment was reorganize and redesignated as the 21st Infantry Regiment. The 2nd Battalion, then stationed at Camp Winthrop near Richmond, Virginia officially became the 21st Infantry Regiment on 7 December 1866. 3 March 1869 Congress directed the War Department to reduce the total infantry regiment to twenty-five. As part of the reduction the 32nd Infantry was consolidatd with the 21st Infantry. The 32nd was serving in the Arizona Territory. The 21st was ordered to Arizona and departed Richmond on 12 April 1869 and became the first troops to deploy from the east coast to California over the just completed Omaha to San Francisco transcontinental rail line. During the period 9-31 August 1869 each company of the 21st moved into Arizona and consolidated with its corresponding company of the 32nd. Military Returns for Camp Bowie cite the following: August 21st Co. D 21st U.S. Infantry arrived at this post, and is now making its consolation with Co. “D” 32nd Infantry (in compliance with G.O. No. 31, Col. 2, Dept Cal May.12.18) stationed at this post for duty. Based on information discovered within the Military Returns for Fort Bowie, Patrick Breen was at Camp Bowie from 21 Aug 1869 until December 1871 when his transfer to join the 1st Cavalry was approved.