The Army of the Potomac was the principal Union army during the American Civil War, formed in 1861. It was responsible for protecting the capital, Washington, D.C., and engaging the Confederate forces in the Eastern Theater of the war. The army played a crucial role in several significant battles, including the Battle of Gettysburg and the Battle of Antietam. Throughout the war, the Army of the Potomac underwent various leadership changes, with notable commanders such as George B. McClellan, Joseph Hooker, and Ulysses S. Grant. The army's efforts were instrumental in achieving Union victories and ultimately contributed to the defeat of the Confederacy in 1865.