At the start of the American Civil War, Poe assisted in organizing the volunteers from Ohio; later, he was made a member of Maj. Gen. George B. McClellan's staff in western Virginia and took part in the Rich Mountain campaign. He went with McClellan to Washington and assisted by organizing the defense of the capital. Promoted to colonel of volunteers that September, he was placed in command of the 2nd Michigan Volunteer Infantry Regiment. He commanded them successfully from Yorktown through the Battle of Seven Pines during the Peninsula Campaign; he next was given field command of a brigade prior to the Northern Virginia Campaign of 1862. His brigade anchored the far right of the Union line at the August 29–30 battle of Second Bull Run but was only lightly engaged. Several days later on September 1, Poe and his men participated in the Battle of Chantilly. His brigade was present, but not active during the December 1862 Battle of Fredericksburg.