Summary from Wikipedia

In 1975, Fischer refused to defend his title when an agreement could not be reached with FIDE, chess's international governing body, over the match conditions. Consequently, the Soviet challenger Anatoly Karpov was named World Champion by default. Fischer subsequently disappeared from the public eye, though occasional reports of erratic behavior emerged. In 1992, he reemerged to win an unofficial rematch against Spassky. It was held in Yugoslavia, which at the time was under an embargo of the United Nations. His participation led to a conflict with the US federal government, which warned Fisch ...