Muhammad Ali, born Cassius Clay on Jan. 17, 1942, is regarded as one of the top professional boxers of all time. He was an activist who spent his life fighting for and enacting change.

Ali went by his birth name for the first few years of his professional career. In 1964, he converted to Islam and took the name given to him by his mentor, Elijah Muhammad. He continued to use that name for the rest of his life. 

Ali, nicknamed "the greatest," began boxing at the young age of 12. He became a member of the 1960 United States Summer Olympic boxing team and won a gold medal in the light heavyweight division. Ali's professional boxing career took off following his victory. 

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Ali fought in a number of widely remembered matches, including a 1964 fight against Sonny Liston for the heavyweight title. Though an underdog in the match, Ali defeated Liston to earn the title. Liston earned the title himself in 1962 after quickly knocking out his opponent, Floyd Patterson. 

Ali's career came to a halt in 1967 after he refused to join the military due to his religious beliefs and opposition to the Vietnam War. 

After Ali's refusal, he was stripped of his titles, but he quickly appealed his conviction. Ali did not go to jail as he awaited a decision, but he was suspended from professional boxing. 

In 1971, the decision was overturned by the Supreme Court, and Ali returned to the ring. 

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Many of Ali's fights are remembered long after his death in 2016.

Ali fought Joe Frazier multiple times throughout his boxing streak. Frazier, the South Carolina-born professional boxer, was the first to beat Ali in a professional match-up. He defeated "the greatest" on March 8, 1971, in the Fight of the Century.

He also went head-to-head with George Foreman in The Rumble in the Jungle. Ali won this 1974 bout to reclaim his title. 

Foreman, a Texas-born professional boxer, was world heavyweight champion from 1973 until the following year, when he lost his title to Ali.

Ali was also an activist who was named the U.N. Messenger of Peace in 1998. He received the Presidential Medal of Freedom in 2005 from former President George W. Bush.