Chess right of a deposed champion to a rematch, and the next champion, Armenian Tigran Petrosian, a player renowned for his defensive and positional skills, held the title for two cycles, 1963–1969. His successor, Boris Spassky from Russia (champion 1969–1972), won games in both positional and sharp tactical style. The next championship, the so-called Match of the Century, saw the first non-Soviet challenger since World War II, American Bobby Fischer, who defeated his Candidates opponents by unheard-of margins and clearly won the world championship match. In 1975, however, Fischer refused to defend his title against Soviet Anatoly Karpov when FIDE did
Bobby Fischer Chess Championship. Therefore, I resign my FIDE World Chess Championship title. Sincerely, Bobby Fischer. The delegates responded by reaffirming their prior decisions, but did not accept Fischer's resignation and requested that he reconsider. Many observers considered Fischer's requested 9–9 clause unfair because it would require the challenger to win by at least two games (10–8). Botvinnik called the 9–9 clause "unsporting". Korchnoi, David Bronstein, and Lev Alburt considered the 9–9 clause reasonable. Due to the continued efforts of US Chess Federation officials, a special FIDE Congress was held in March 1975 in Bergen, Netherlands, in which it was accepted that