Muhammad Ali was an American professional boxer, generally regarded as the most significant heavyweight in the history of the sport. Early in his career, Ali was known for being an inspiring, controversial and polarizing figure both inside and outside the boxing ring. He is one of the most recognized sports figures of the past 100 years, crowned "Sportsman of the Century" by Sports Illustrated and "Sports Personality of the Century" by the BBC. He also wrote several best-selling books about his career, including The Greatest: My Own Story and The Soul of a Butterfly. Ali began training at 12 years old. At the age of 22, he won the world heavyweight championship from Sonny Liston in a stunning upset in 1964. Shortly after that, Ali joined the Nation of Islam and changed his name. He converted to Sunni Islam in 1975, and then to Sufism in 2005. Ali retired from boxing permanently in 1981. In 1967, three years after winning the heavyweight title, Ali refused to be conscripted into the U.S. military, citing his religious beliefs and opposition to American involvement in the Vietnam War. He was eventually arrested and found guilty on draft evasion charges and stripped of his boxing title. He did not fight again for nearly four years-losing a time of peak performance in an athlete's career. Ali's appeal worked its way up to the Supreme Court of the United States where, in 1971, his conviction was overturned. Ali's actions as a conscientious objector to the war made him an icon for the larger counterculture generation. Ali remains the only three-time lineal world heavyweight champion; he won the title in 1964, 1974, and 1978. Between February 25, 1964 and September 19, 1964 Ali reigned as the undisputed heavyweight boxing champion. Nicknamed "The Greatest," he was involved in several historic boxing matches. Notable among these were the first Liston fight, three with rival Joe Frazier, and "The Rumble in the Jungle" with George Foreman, in which he regained titles he had been stripped of seven years earlier.