During the American Civil War, the Commonwealth of Pennsylvania played a critical role in the Union, providing a substantial supply of military personnel, equipment, and leadership to the Federal government. The state raised over 360,000 soldiers for the Federal armies. It served as a significant source of artillery guns, small arms, ammunition, armor for the new revolutionary style of ironclad types of gunboats for the rapidly expanding United States Navy, and food supplies. The Phoenixville Iron Company by itself produced well over 1,000 cannons, and the Frankford Arsenal was a major supply depot.

The Gettysburg campaign was a military invasion of Pennsylvania by the main Confederate army under General Robert E. Lee in summer 1863. It was the first time during the war the Confederate Army attempted a full-scale invasion of a free state. The Union won a decisive victory at Gettysburg, July 1–3, with heavy casualties on both sides. Lee managed to escape back to Virginia with most of his army. It was a turning point in the American Civil War, with Lee increasingly pushed back toward Richmond until his surrender in April 1865. The Union Army of the Potomac was commanded by Maj. Gen. Joseph Hooker and then by Maj. Gen. George G. Meade.