He served in the Pennsylvania Reserves during the first part of the war and later commanded brigades in western Virginia and at Petersburg, where a serious wound ended his military career.

In 1861 Sickel volunteered for the Union army and was appointed captain (May 27) then colonel (July 28) of the 3rd Pennsylvania Reserves.

[2] Sickel and his regiment joined the Army of the Potomac on the Virginia Peninsula in time for the Seven Days Battles.

While many of the Pennsylvania Reserves rejoined the Army of the Potomac for the Gettysburg campaign, Sickel stayed behind as commander of a brigade in the Department of Alexandria.

At the Battle of Cloyd's Mountain, he led his men in a charge against the Confederate breastworks which succeeded in turning the enemy's flank.

[1] Sickel took a supporting role in the Battle of Boydton Plank Road[3] but was again significantly engaged at Hatcher's Run.

At the Battle of Quaker Road, Sickel "greatly distinguished himself", according to Chamberlain, "...and [behaved] in the most admirable manner.