Ali, originally known as Cassius Clay, began training at 12 years old and at the age of 22 won the world heavyweight championship in 1964 from Sonny Liston in a stunning upset. Shortly after that bout, Ali joined the Nation of Islam and changed his name. He converted to Sunni Islam in 1975, and 30 years later began adhering to Sufism.
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. Ali's appeal worked its way up to the U.S. Supreme Court, 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 Muhammad Ali reigned as the undisputed heavyweight boxing champion.