By 1967, Muhammad Ali was the reigning heavyweight boxing champion of the world, at the peak of both his athletic ability and his fame. That year, he was drafted into the United States military to fight in the Vietnam War, a war he publicly opposed on both religious and moral grounds after converting to Islam.
He refused induction into the army, stating plainly that he had no personal quarrel with the Vietnamese people and would not fight a war he believed was unjust, regardless of the consequences to his own career. He knew, going in, exactly what it would cost him.
The consequences were severe and immediate: he was stripped of his boxing title, banned from fighting in the United States, convicted of draft evasion, and had his passport revoked, all while he was in the physical prime of his career, roughly three and a half years during which he could not box professionally at all.
His conviction was eventually overturned by the U.S. Supreme Court in 1971, and he returned to boxing, going on to reclaim the heavyweight title multiple times through his thirties. Decades later, he was widely regarded not just as one of the greatest boxers in history, but as someone who had knowingly sacrificed his athletic prime for a principle.