Due to the number of camps that surrounded it, its secure rearward location, and multiple telegraph connections with Washington, Alexandria, and other forts defending Washington, Fort Corcoran became the headquarters for Washington's defenses south of the Potomac River. On September 6, 1862, General Fitz John Porter assumed command of those defenses and made his headquarters at Fort Corcoran. He was only at Fort Corcoran for a short time before he and the rest of his command were withdrawn from the defenses of Washington in order to counter Lee's advance into Maryland. A bare handful of artillerymen were left to man the guns of the forts along the Arlington Line. Following the climactic Battle of Antietam, Porter and his men returned to Fort Corcoran. A few months following Antietam, Porter was court-martialed for his actions at the Second Battle of Bull Run and was removed from command.