Unification of the World Chess Champion title became an issue in the mid-1990s. In 1993, then World Champion Garry Kasparov left FIDE to play a match against Nigel Short, who had emerged as Kasparov's challenger in the FIDE qualifying cycle.
The players created the Professional Chess Association (PCA) and played a match which Kasparov won. The PCA set up a system of qualifying events similar to the FIDE system. Meanwhile, FIDE continued to organize its own series of World Championships, eventually adopting a knockout format. This left the world with two World Champions.
After a few years of initial success, the PCA ran into difficulties with sponsorship. Talks between the PCA and FIDE began to revert to the old system of a single championship. This process, known as unification (or reunification) bogged down in procedural details like who would win in case of a drawn match.
In 2002, all parties signed the Prague Agreement, a process drafted by GM Yasser Seirawan to culminate eventually in a single World Champion. By 2005, the Prague Agreement had been largely scrapped.