What inspired the person who wrote national anthem to write it?

Francis Scott Key wrote the "Star-Spangled Banner" after witnessing the Bombardment of Fort McHenry by British ships in September 1814. Key was detained on a British ship during the conflict. The next morning, when Key saw that the American flag was still flying over the fort, he wrote a poem called "The Defense of Fort M'Henry" in a fit of patriotic elation. After this poem made its way onto a printer's sheet, it gained popularity as a song titled "The Star-Spangled Banner." On March 3, 1931, President Hoover signed a Public Law designating "The Star-Spangled Banner" as the national anthem of the United States.