Major League rivals to the NFL have existed from the 1920s. Various leagues from 1920s to 1960s called themselves the American Football League, and lasted usually a couple of years before one in 1960 was successful enough to force a merger with the NFL, which is why the Super Bowl is played – between the winners of the two leagues (now AFC and NFC)
Various Leagues have since then tried the same route, though with different outlooks – such as being a World League, Spring League, Indoor League or Extreme League.
Major Leagues (AFL Era):
Rival Major Leagues (Modern Era):