The ubiquitous song, which is played everywhere from sporting events to patriotic rallies, started out as a poem written by a lawyer, Francis Scott Key. Key was there as part of a team trying to negotiate the release of prisoners when the Battle of Baltimore broke out. He was so inspired by seeing the then 15-star American flag of Fort McHenry waving during the bombardment that he wrote his poem honoring the flag.
Later on Key's brother-in-law, Joseph H. Nicholson, read the poem and recalled an old drinking song called "To Anacreon in Heaven." He felt Key's words matched well with the tune and thus a new song was born. It wouldn't be until over 100 years later though that the song would be officially adopted as the national anthem.
Continue learning about Francis Scott Key and how "The Star-Spangled Banner" came to be our national anthem at Ammo.com: https://ammo.com/articles/star-spangled-banner-forgotten-history-americas-national-anthem-francis-scott-key