Armory Square Hospital, located in Washington, D.C., was a military hospital established during the U.S. Civil War. It served as a major treatment center for Union soldiers injured during battles, particularly those from the Battle of Antietam and the Peninsula Campaign. The hospital complex spanned several buildings, including the Armory Building, Old Congressional Cemetery, and the Patent Office Building. Its significance lies in its role as one of the busiest hospitals during the Civil War, providing care for thousands of soldiers and contributing to the medical advancements of the time. Today, remnants of the hospital complex can be found in various parts of Washington, D.C., serving as a testament to the city's history and the challenges faced during the U.S. Civil War.