In 1861 Sickel volunteered for the Union army and was appointed captain (May 27) then
colonel (July 28) of the 3rd Pennsylvania Reserves. Sickel and his regiment joined the
Army of the Potomac on the Virginia Peninsula in time for the
Seven Days Battles. He led his regiment credibly at
Mechanicsville and
Gaines' Mill, receiving commendations from his brigade and division commanders,
George G. Meade and
George McCall. He immediately reenlisted and was appointed colonel of the
198th Pennsylvania Infantry. This command was ordered to the Petersburg front and upon arrival there, Sickel was placed in command of the 1st Brigade in the 1st Division of the
V Corps. Two weeks later, Sickel led his brigade into action at
Poplar Springs Church. For his services there, he was given a
brevet promotion to
brigadier general of volunteers on October 21, 1864. but was again significantly engaged at
Hatcher's Run. There, he received a painful flesh wound in his left thigh on February 6, 1865. This wound bothered him for the remainder of his life. In November 1864, General
Joshua L. Chamberlain, permanent commander of the 1st Brigade, returned to duty, and Sickel reverted to regimental command. He was given a brevet promotion to
major general on March 13, 1865. At the
Battle of Quaker Road, Sickel "greatly distinguished himself", according to Chamberlain, "...and [behaved] in the most admirable manner." ==Later life==