CINCINNATI, OHIO • The upstart Federal League‘s attempt at being a third major league came to an end on this date in 1915. The official word was National League, American League and Federal League bosses settled their differences at a meeting in Cincinnati. What in fact happened was the NL and AL flexed their muscles, and the Federal League ceased to exist. The rise and fall of the renegade league also put the wheels in motion to exempt major league baseball from competition.
The Federal League came about as a minor league in 1912. It declared itself a “major league” in 1914 and had a couple successful seasons with close pennant races, stars lured from the National and American Leagues and good attendance. It was an eight-team league competing in the major league cities of Chicago, St. Louis, Brooklyn and Pittsburgh (it also had teams in Baltimore, Indianapolis, Kansas City and Buffalo).
What brought about the events of this day in 1915 was the Federal League had filed an antitrust lawsuit against the National and American Leagues claiming they were illegal monopolies. The case stalled in the court of federal judge Kennesaw Mountain Landis while the future baseball commissioner urged negotiation. The Federal League’s position weakened as the delay drained it of funds. Several FL owners were bought out and some teams absorbed into the NL and AL.
But the Baltimore franchise of the Federal League was not happy with the agreement and sued. The lawsuit went all the way to the United States Supreme Court which ruled in 1922 that major league baseball was exempt from the Sherman Antitrust Act, a decision in affect to this day.
Ironically, the episode gave a glimpse of what was to come 60 years later – free agency. Not only would the 1915 agreement bring amnesty for National and American League players who had jumped to the Federal League, but they would be able to sell their services to the highest bidder.
Another legacy of the defunct Federal League was Chicago’s Weeghman Park, 
built for the now defunct Chicago Whales. It was taken over by the National League franchise Chicago Cubs and renamed Wrigley Field, the same park they play in to this day.
Contributing sources:
Atlanta Constitution, Atlanta, Georgia, December 23, 1915
Wikipedia