 Complete the following phrase:  In 1966, three years after Petrosian had earned the title of World Chess Champion, he was challenged by Boris Spassky. Petrosian defended his title by winning rather than drawing the match, a feat that had not been accomplished since Alexander Alekhine defeated Efim Bogoljubov in the 1934 World Championship. However, Spassky would defeat Efim Geller, Bent Larsen, and Viktor Korchnoi in the next candidates cycle, earning 