In 1967, Muhammad Ali refused to allow himself to be drafted into the United States Army, in protest of the Vietnam War and the state of racial oppression in the United States. At the time he was the undefeated heavyweight champion, but as he was plunged into a lengthy legal battle, the various athletic commissions in the country refused him licences to fight and forced him into retirement. 

If he had never returned, he would have gone down as among the best champions in the history of the division. But what he did after finally being able to return in 1970 made him the popular choice for the best big man of all time and the most popular professional athlete of the late 20th century. 

The heavyweight division in the 1970s was the most competitive it has ever been. Ali's first two fights after his three-year break were stoppages of Jerry Quarry and Oscar Bonavena, contenders who could have been champions in other eras. 

This set up "The Fight of the Century" between Ali and the undefeated champion Joe Frazier in March 1971. Frazier dropped Ali with a beautiful lead hook in Round 15 and won by unanimous decision.

Ali remained among the top fighters in the heavyweight division, but by 1973, it began to look unlikely that he would ever regain the crown. In February of that year, he got his jaw broken and lost by split decision to Ken Norton.

While Ali won a rematch later in the year, 1973 was the year that Big George Foreman emerged. In January of that year he destroyed Joe Frazier in two rounds to claim the heavyweight belt.

Ali won a rematch with Frazier in January 1974, to set up his shot at Foreman. The fight was held in Zaire in October 1974 and was the center of a multi-day music festival. It was a huge international event that boxing could only dream of today.

Fighting a bigger, younger man who was thought to be unbeatable, Ali used guile, heart and psychology to break Foreman and knock him out in Round 8.

Ali lost the title to upstart Leon Spinks in 1978 then won it back in an immediate rematch, to become the first three-time heavyweight champion in history. He then retired but launched two more unfortunate comebacks, getting pounded by Larry Holmes in 1980 and losing via unanimous decision to Trevor Berbick in 1981.