While soccer is probably the most popular sport in the world, football is certainly the most popular sport in America. Football has been played in the United States since the 1800s, and interest in amateur and college football teams eventually led to the formation of professional leagues.
Officially, the National Football League was formed in 1920. The teams that constituted the new league were already playing as professionals; they just hadn’t played each other in a formal season that ended in a title, and eventually, a title game to decide who was champion.
The NFL as it’s currently constituted wasn’t formed until 1970, when the existing NFL merged with the newer American Football League, with the resulting league deciding the championship between conferences with a season-ending Super Bowl, a game that has become the most famous single-day sporting event in the world.