Where Did The US National Anthem Come From?

It was originally a drinking song.  

Francis Scott Key, a 35 year old lawyer and part-time poet, wrote the words to the "Star Spangled Banner" in 1814.  Key was briefly detained on a British ship, part of a group of ships bombarding an American fort, and was so pleased to see the US flag still flying the next morning that he wrote the poem.

Key published his poem soon after he wrote it, and then set it to the same tune as a popular drinking and erotic song of British origin, To Anacreon in Heaven.    The original lyrics are mostly about sex and the joys to be found therein.

It's thought that the original tune was written about 1750, by an unknown English composer.  Everyone agrees that the song is challenging to sing properly.

The US Congress made the Star Spangled Banner the official US national anthem in 1931, although it had already been used as such by the US military forces since the 1880s, and at White House functions since 1916.