Footballhttp://en.wikipedia.org/wiki/AFL–NFL_merger
At first, the NFL ignored the AFL and its eight teams, assuming the AFL would consist of players who could not earn a contract in the NFL, and that fans of professional football would not waste their time watching them when they could watch the NFL. The NFL also had the media advantage. For example, in the 1960s, Sports Illustrated's lead football writer was Tex Maule, whose previous job had been as public relations director for Pete Rozelle, the general manager of the NFL's Rams. Maule "was certainly an NFL loyalist,"[1] and several sports reporters took his deprecatory columns about the AFL as fact. In another example, another former Rozelle employee, Tex Schramm, was CBS's director of sports during the period when that network refused to give AFL scores. Many play-by-play and color announcers on CBS were former NFL players.
However, in spite of this bad press, and unlike the NFL's previous rivals, the AFL was able to survive and grow. After the league's Los Angeles team moved to San Diego (in 1961) and the Dallas team moved to Kansas City (in 1963), the league began to prosper. The New York team (now called the Jets) began to draw record crowds, aided by the signing of quarterback Joe Namath to an unprecedented $427,000 contract. NBC paid the AFL $36 million in 1965 to televise its games, ensuring the league's financial survival.
As the rivalry between the leagues intensified,
both leagues entered into a massive bidding war over the top college prospects, paying huge amounts of money to unproven rookies in order to outbid each other for the best players coming out of college.
Contrary to common belief, it was not the AFL, but the NFL that initiated discussions for a merger between the two leagues, as it was fearful that Davis' "take no prisoners" tactics would seriously reduce its talent base.
Baseballhttp://en.wikipedia.org/wiki/American_LeagueThe American League of Professional Baseball Clubs, or simply the American League (AL), is one of two leagues that make up Major League Baseball in the United States and Canada. It developed from the Western League, a minor league based in the Great Lakes states, which eventually aspired to major league status. It is often called the Junior Circuit because it claimed Major League status for the 1901 season, 25 years after the formation of the National League (the "Senior Circuit").
http://en.wikipedia.org/wiki/Federal_LeagueThe Federal League came together in early 1913 through the work of John T. Powers, and immediately challenged the operations of organized baseball. Playing in what detractors called the "outlaw" league allowed players to avoid the restrictions of the organized leagues' reserve clause.
The competition of another, better paying league caused players' salaries to skyrocket, demonstrating the bargaining potential of free agency for the first time.
Hockeyhttp://en.wikipedia.org/wiki/NHL–WHA_merger
In its seven seasons, the WHA paid its players $120 million, and lost over $50 million.
The competition for talent introduced by the WHA, and accelerated by the signing of Bobby Hull, led to a rapid escalation of salaries for players in both leagues. Hockey players had leverage in contract negotiations for the first time.
Basketballhttp://en.wikipedia.org/wiki/ABA–NBA_merger
From the very beginning, the ABA hoped to force a merger with the NBA, thus repeating the American Football League's successful effort to force a merger with the National Football League. According to The NBA Encyclopedia,
the ABA's main selling point was that a potential owner could get an ABA team for half of what it cost to get an NBA expansion team at the time. ABA officials told prospective owners that if they held out until the merger, their investment would more than double.