The World Football League (WFL) started in 1974 and had experienced serious financial difficulties. Before the start of their second season, the Memphis Southmen of the WFL signed Dolphins players Larry Csonka, Paul Warfield, and Jim Kiick away from Miami to play for Memphis. The Memphis franchise was one of just a couple WFL teams that had semi-smooth operations, eventually petitioning to join the NFL after the WFL folded but they were denied.
The effect this mass evacuation of players had on the Dolphins was significant and the AFC would’ve been significantly different had these players not gone.
It would be as if Troy Aikman, Michael Irvin, and Emmitt Smith had left the Cowboys in the early 1990’s. Peyton Manning, Marvin Harrison, and Edgerin James leaving the Colts in the early 2000’s, would’ve had a similar effect on their team.
Alternative professional leagues have had a wide variety of effects on the National Football League, but few have had such a dramatic effect on one specific team.