Francis Scott Key, the lawyer, was also an amateur poet. He was so overwhelmed by what he had seen during the battle, and so relieved by what he had seen at first light, he wrote down some words on the back of a letter he had in his pocket.
The next day, on September 16, when he and the other two men were allowed ashore, Key took a room at the Fountain Inn. There he revised the words he had drafted while aboard ship. He showed the poem to his brother-in-law who gave it to the Baltimore Patriot. The Patriot published the poem on September 20, 1814. It was soon published in other newspapers in other states.
The following month, an actor sang those now-famous words using the tune of To Anacreon in Heaven a popular drinking song composed by John Stafford Smith around 1775. The song (named in honor of a poet, Anacreon, who lived in Greece between 563-478 B.C. and wrote of love and wine) was sung in London during meetings of a "gentlemen’s society." (You will need RealAudio for this link.)
Ever since Key’s poem was set to music, the song was called The Star-Spangled Banner. The first verse became America’s de facto national anthem. It became the official anthem when Congress passed a law to that effect in 1931.