The Sports Interactive/Eidos split occurred in late 2003, when the creators and developers of the Championship Manager series, Sports Interactive and the publishers of the series, Eidos decided to go their separate ways.
Massive uncertainty followed and some of the more pessimistic CM fans thought that the series had come to an end. As part of the split, SI were allowed to keep the rights to the database and the game's code, whereas Eidos kept the Championship Manager name and the so-called "look and feel" of the game.
SI vowed to continue developing management games and set about finding a new name for their iconic series. It wasn't long before SI revealed that they had purchased the rights to the name "Football Manager", a well-respected series of management games from the 1980's.
Eidos, on the other hand, were left with the Championship Manager name but no game to support it. They set up a new development team, Beautiful Game Studios, specially to create a worthy successor to SI's legendary series. For a while, fans were undecided about which side their loyalty lay with; the name they had grown so familiar with, or the team that had created and evolved the games behind it. Sports Interactive stole a march on Eidos when they released their first of the new Football Manager series ahead of schedule, with Eidos missing thier release date and unveiling Championship Manager 5 several months late, and to luke warm reviews.
Football Manager, to this day, continues to lead the way in terms of popularity and sales, well ahead of all its competitors, including the new-look CM.