Boston bm.  In May 1861 he returned to U.S. military service as 1st Lieutenant in the 1st U.S. Cavalry Regiment. He was part of the regular Cavalry service until he was appointed Colonel of the 1st Massachusetts Volunteers Infantry Regiment. In this capacity he fought at Fredericksburg, Chancellorsville, Gettysburg, Mine Run, Wilderness and Spotsylvania. He won brevet promotions to major in the U.S. Army for Chancellorsville and to lieutenant colonel for Gettysburg.[1] He was mustered out of the volunteers service on May 28, 1864.[2]Siege of PetersburgOn September 14, 1864 he was appointed Colonel of the 57th Massachusetts Infantry[2] returning to the front lines during the Siege of Petersburg. The following day McLaughlen assumed command of the 3rd Brigade, 1st Division, IX Corps. He led it at the Battle of Poplar Springs Church for which he was given a brevet promotion to brigadier general of U.S. Volunteers, and also fought at the Battle of Boydton Plank Road. During the Winter of 1864-1865 McLaughlen was occasionally in command of the 1st Division, IX Corps.[3]Fort StedmanMcLaughlen returned to command of the 3rd Brigade which manned the line of trenches constituting Fort Stedman. On March 25, 1865 Confederate General John B. Gordon launched a surprise attack against Fort Stedman, overrunning its defenses. McLaughlen moved to the front to rally his brigade. He first inspected Fort Haskell, another defense in his sector. Approving of the situation at Fort Haskell he rode on to Fort Stedman not realizing this position was now occupied by Confederate soldiers, who took him prisoner. Despite his capture he was given a brevet promotion in the U.S. Army to colonel for Fort Stedman, then brigadier general. He was briefly held prisoner in Libby Prison[1] until his release on April 2, 1865. He returned to command his old brigade which was now manning the defenses of Washington, DC before he was mustered out of the volunteer service on August 10, 1865.[citation needed]