Here are some quotes from King of the World by David Remnick:

As Cassius Clay, he entered the world of professional boxing at a time when the expectation was that a black fighter would behave himself with absolute deference to white sensibilities, that he would play the noble and grateful warrior in the world of southern Jim Crow and northern hypocrisy (xiii).

Ali began fighting when the ideas about how African Americans should behave as public athletes changed from being submissive to being less so, and more politically involved.

Clay was mightily aware that his gold medal changed nothing about Louisville. The same old Jim Crow attitudes still prevailed (106).

Even though Ali won a gold medal in the 1960 Olympics, his hometown was still plagued by racism; he believed that the U.S. had to change.

Even Ali, who would earn millions of dollars in the ring, who became famous and adored because of his skill at beating other men, even he expressed ambivalence about the spectacle of two black men fighting (221).

The history of black fighters started in slavery, when white men liked to pit black men against each other. Ali was aware of this racist legacy.

Boxing as a racial metaphor intensified in the sixties. And even though Ali may not have read every article written about him, he was deeply aware of his position in relation to both Jack Johnson and Joe Louis (228).

Ali came to prominence after black fighters, such as Jack Johnson, who had been punished for defying racial stereotypes.

That was enough to confirm all the stories in the press: Clay was a member of the Nation of Islam. But whether the press understood it or not, he had quietly forsaken the image of the unthreatening black fighter established by Joe Louis and then imitated by Jersey Joe Walcott and Floyd Patterson and dozens of others (207).

Ali challenged the idea of the non-challenging black fighter and became decidedly confrontational and political.