What’s unusual about the music to the American national anthem?

In 1814, after a night in a pub, Francis Scott Key was taken prison-
er during the war between Canada and the United States. When
he saw the American flag still flying over Fort McHenry he
was inspired to write his famous lyrics with one particular barroom
song, “To Anacreon In Heaven,” still in his mind. And so “The
Star Spangled Banner” was written to the tune of a traditional
old English drinking song.