Encyclopedia Smithsonian: Star-Spangled Banner and the War of 1812

When he saw “by the dawn’s early light” of September 14, 1814, that the American flag soared above the fort, Key knew that Fort McHenry had not surrendered.  Moved by the sight, he began to compose a poem on the back of a letter he was carrying.

via Encyclopedia Smithsonian: Star-Spangled Banner and the War of 1812.