On September 14, 1814, Francis Scott Key, a 35-year-old Washington lawyer, wrote a four-stanza poem on the back of a letter he had in his pocket. He had spent the night on a truce ship in the Patapsco River watching the British bombard Fort McHenry, and in the clearing weather that morning he could see the flag still up. The poem was called Defence of Fort McHenry. It became the national anthem 117 years later.
Key was not a soldier and had not meant to be there. He was there by accident. He had gone down to the British fleet to negotiate the release of a friend, Dr William Beanes, taken after the British marched into Washington and burned the public buildings in August. Key and Colonel John Skinner got Beanes released but were held on board while the fleet prepared to attack Baltimore, because they had seen the preparations and heard the plan.
The bombardment ran for about 25 hours, from the morning of September 13 into the night and into the early hours of the 14th. The British fired around 1,500 shells and Congreve rockets at the fort, at a range just outside the effective reach of their own guns and, more usefully, outside the reach of the fort's. The garrison had about 1,000 men, under Major George Armistead. It lost four killed. The fort held.
The flag Key saw was not the one on the fort today. It was a garrison flag, 30 by 42 feet, with fifteen stars and fifteen stripes, made in Baltimore by Mary Pickersgill the previous year at the request of Armistead, who had asked for a flag large enough that the British would have no trouble seeing it. It is now in the Smithsonian's National Museum of American History.
Key set his words to an existing melody, To Anacreon in Heaven, a song by John Stafford Smith that was the tune of a London gentlemen's club and was already popular in America. The song was printed within a week as a broadside under the name Defence of Fort McHenry, and it was later renamed The Star-Spangled Banner. Congress made it the national anthem on March 3, 1931, more than a century after it was written.
The Battle of Baltimore, which it describes, was a turning point of the War of 1812: the British landed at North Point, where their commander Robert Ross was killed, and with the fort holding they withdrew. Peace was signed at Ghent in December.