Muhammad Ali was one of the most significant sports figures of the 20th century. He was born Cassius Marcellus Clay Jr. in Louisville, Kentucky, in 1942. At 18, he won a gold medal in the light heavyweight division at the 1960 Summer Olympics and turned professional later that year. He won the world heavyweight championship in 1964, defeating Sonny Liston in a major upset. In 1966, Ali refused to be drafted into the military owing to his religious beliefs and ethical opposition to the Vietnam War. He retired from boxing in 1981 and focused on religion, philanthropy and activism.