If I might be a bit pedantic, Key didn't write a national anthem. He wrote a patriotic song about his country, one of many that cropped up during wartime, and it was made the national anthem over a century later. Prior to 1931, America had no official national anthem, although there were several songs that served the same purpose unofficially, most commonly Samuel Francis Smith's America (also called My Country, 'Tis of Thee).