On September 13, 1814, the young American lawyer Francis Scott Key was inspired by the successful American defense of Fort McHenry, Maryland, during the War of 1812. The poem he wrote about the battle, “Defense of Fort M’Henry,” was put to music and eventually became the national anthem of the United States, “The Star-Spangled Banner.” Key witnessed the battle while he was briefly detained on British gunboats in Baltimore Harbor, where Fort McHenry is located. Key was negotiating for the release of some American prisoners, and the British temporarily held him after he overheard their plans to attack Baltimore. From the British ships, Key saw the fort’s 5x8 meter (17x25 foot) “storm flag” hit with British rocket fire. Key didn’t know the outcome of the battle until the following morning, when the fort unfurled its massive 9x13 meter (30x42 foot) banner. Set to an already popular tune, “Defense of Fort M’Henry” became one of the most popular patriotic songs of the 19th century. It was officially declared the national anthem in 1931.