On February 1 1960, four young black men walked into Woolworths in Greensboro, North Carolina, and demanded service at the whites-only counter. Independent of their elders and unconcerned by the prospect of imprisonment, they set the tone for a decade of protest and a fundamental shift in the consciousness of black Americans.

"On the day that I sat at that counter, I had the most tremendous feeling of elation and celebration," says one of the protesters, Franklin McCain, who was 17 at the time. "I felt that in this life nothing else mattered - I just felt that you can't touch me. You can't hurt me."

By the end of the year, halfway across the world, Togo, Zaire, Somalia, Benin, Niger, Burkina Faso, Cote d'Ivoire, Chad, Congo Brazzaville, Gabon, Senegal, Mali, Central African Republic, Mauritania and Nigeria had all declared their independence from colonial rule.

In between, a fresh-faced 18-year-old from Louisville, Kentucky, called Cassius Clay strutted around the Olympic village in Rome, where his boisterous patter and amiable character would earn him the nickname "the mayor", telling everyone who would listen that he would be the greatest boxer of all time. Clay left the games with the light-heavyweight gold medal. He had taken his first step to becoming a global icon.

"Men make their own history," wrote Karl Marx. "But they do not make it just as they please; they do not make it under circumstances chosen by themselves, but under given circumstances directly encountered and inherited from the past."

Muhammad Ali, as Clay would later become known, certainly made his own history. As the upcoming Will Smith film Ali shows, his personality, morality, humanism, physical strength and boxing technique were all his own. But it was the political, racial and international circumstances in which he would employ those talents that played a crucial role in shaping his own inspiring narrative.

He emerged alongside the anti-colonial movements throughout Africa and Asia and the civil rights movement in America. His political and racial consciousness developed alongside the radicalisation of Black Power and the anti-Vietnam protesters. He was both impacted by them and had an impact on them. Ali was part of the call and the response; both subject and object.

"Once liberated from its parochial prison," writes Mike Marqusee in his landmark book, Redemption Song, "the sixties [were] about the growth of global consciousness from below. For people all over the world Ali embodied that consciousness. It was Ali's transgression of American norms - in an American idiom - that enabled him to build his global constituency."

"What do they know of cricket who only cricket know?" asked Trinidadian-born intellectual CLR James as he wove his understanding of race, class, caste, culture and politics into the history of West Indian cricket. The same question can be asked of boxing, which for the best part of the last century transcended sport. Race was not always the defining factor. In 1889, when John L Sullivan defended his bare knuckle title against Jake Kilrain, immigration was the faultline. Sullivan was portrayed as the bad Irish immigrant, the drinker and womaniser, pitted against the diligent and virtuous Kilrain.

But with a new century came new priorities. "The problem of the 20th century is the problem of the colour line," wrote African-American intellectual and civil rights leader WEB DuBois in 1900. And so it was that one could trace US anxieties and prejudices about race and nationality through its attitudes to champions and contenders, straight to Ali.

When Jack Johnson emerged as the first black heavyweight champion from his bout with the Irishman Tommy Burns in 1908, former champion Jim Jeffries came out of retirement to claim the mantle: "I'm going into this fight for the sole purpose of proving that a white man is better than a Negro," he said. Johnson did not only have the audacity to win the title, he also openly cavorted with white women. "If the black man wins," warned the New York Times, "thousands of his ignorant brothers will misinterpret his victory as justifying claims to much more than physical equality with their white neighbours." When Jeffries went down in the 15th round, rioting erupted across the country.

Johnson lost the title in 1915 and no black challenger was allowed to compete for it again - Jack Dempsey refused to fight blacks - until the 1930s. By then Joe Louis was on the scene. In 1935 Louis took the title, defeating Italian-American Primo Carnera. "Champion of the world. A black boy. Some black mother's son," recalled African-American poet and playwright Maya Angelou in her autobiography. Along with the rest of the black community of Stamps, Arkansas, she had listened to the match on the radio in her mother's shop. "Those who lived too far had made arrangements to stay in town. It wouldn't do for a black man and his family to be caught on a lonely country road on a night when Joe Louis had proved that we were the strongest people in the world."

When Louis went on to fight German challenger Max Schmeling in 1936, nationalism trumped race. The press embraced Louis as a patriot who knew his place where racial politics were concerned. In later years he would proudly join the army, which was segregated at the time, and donate the winnings from one fight to the Navy Relief Fund.

Louis lost to Schmeling in 1936, only to restore American pride by defeating him two years later. "There never has been a heavyweight champion who has behaved better than Joe has, in and out of the ring," said the Chicago Tribune. "There are those who say he isn't very bright, but he is bright enough to know where his duty lies."

More than a decade later would come Floyd Patterson, a mild-mannered integrationist described by the president of the US national boxing association at the time as "a fine representative of his race". His challenger Sonny Liston, however, who had a long criminal record and few social skills, was demonised. He was "the big black Negro in every white man's hallway, waiting to do him in", wrote Norman Mailer, "[waiting to] deal him under for all the hurts white men, through their arbitrary order, have been able to inflict on the world".

These caricatures were of course as crude as they were offensive. Each of the boxers was his own complex person, full of qualities and contradictions that transcended the limiting confines of race. In the public arena, none the less, each found himself playing a role dictated by the prevailing racial discourse of the day.

Shortly after racist neighbours forced Patterson to leave a new house he had just moved into in a white area in northern Yonkers, he said: "I am just part of the social history of our time and our country and I can't lag behind it - or run too far ahead of it."

Ali was also part of the social history of his time; he just refused to let mainstream society set the pace. Asked by one commentator what kind of champion he intended to be after he beat Liston in 1964, Ali replied: "I don't have to be what you want me to be. I can be what I want to be and I'm free to think what I want to think." It was a powerful statement for a young black man to make, not only in the US but just about anywhere at the time.

It was a statement that could find meaning in Mississippi, where civil rights workers would risk and sometimes lose their lives fighting segregation; but equally in the newly independent and soon-to-be independent states in Africa and the Caribbean. It was the year Nelson Mandela was sentenced to life imprisonment for opposing apartheid and Martin Luther King was awarded the Nobel peace prize for doing the same thing. This was one of the few times last century when the interests of African-Americans and the interests of the broader diaspora converged. With different means and different results in vastly different circumstances, they were all fighting for the vote and civil rights.

"Sure, we identified with the blacks in Africa," said John Lewis, then a leader of the Student Nonviolent Coordinating Committee and now a congressman in Atlanta. "And we were thrilled by what was going on. They were getting their freedom and we still didn't have ours in what we believed was a free country. We couldn't even get a hamburger and a Coke at the soda fountain. What was happening in Africa, finally, had tremendous influence on us." With his visits to Ghana and tours of other African states, Ali encapsulated that mood long before he met George Foreman in Kinshasa for the Rumble in the Jungle.

L ike many young African-Americans, Ali started the decade as a model, patriotic citizen. He was managed by a white syndicate of planters and businessmen from Louisville. When questioned by a Soviet reporter during the Olympics about racism in America, Clay answered: "To me, the USA is still the best country in the world, counting yours."

The path from there to his announcement that he had joined the black separatist organisation the Nation of Islam was a gradual one. Invited to a meeting in Miami he became interested enough to start reading the Nation's paper, Muhammad Speaks. He bought an LP, entitled A White Man's Heaven Is a Black Man's Hell, released by the minister Louis X, the former calypso singer once called The Charmer and today known as Louis Farrakhan.

In 1962, Ali joined the Nation, converted to Islam and became friends with Malcolm X, but did not tell the press. The Nation was happy to leave it that way: its leader, Elijah Muhammad, feared Ali would lose to Liston.

The morning after he defeated Liston, two years later, Ali went public. "I'm no troublemaker," he told the press. "I have never been to jail. I have never been to court. I don't join any integration marches. I don't pay attention to all those white women who wink at me. I don't carry signs. A rooster crows only when it sees the light. Put him in the dark and he'll never crow. I have seen the light and I'm crowing."

The statement would cost him dearly. The World Boxing Association suspended him for "conduct detrimental to the best interests of boxing" and moved to strip him of his title. From then on he would have a hard time not only endorsing products and making other monies out of the ring, but finding promoters who were prepared to put him in it. One evening he was the newly crowned world champion; by the next afternoon he was treated like an outcast. The boxing establishment was livid. Abe Green, commissioner of the WBA, said Ali had to choose between the Nation and his career. "Clay should be given a chance to decide whether he wants to be a religious crusader or the heavyweight champion."

But reaction from the black leaders and boxers was hostile too. Martin Luther King said: "When Cassius Clay joined the Black Muslims he became a champion of racial segregation and that is what we are fighting against." Ali's father accused the Nation of "ruining" his son.

Whatever else it meant, Ali's membership of the Nation did not suggest that, after a decade of civil rights protest, African-Americans were now keen on segregation. He came onboard just as it was about to move into precipitate decline. Less than a month after he joined, Malcolm X left, sparking a vicious internal feud that would end in his assassination and the demise in influence from which it never recovered.

But its symbolic significance could not be underestimated. It came, after a decade of civil rights protest, as black America was contemplating its future strategy. The focus was moving from civil rights to economic rights, from the south to the north and from rural to urban. The following year President Johnson signed the civil rights act, to great acclaim. Within a week of him doing so, race riots broke out in the Los Angeles suburb of Watts.

The Nation had no interest in negotiating a settlement with the white American establishment. For Ali to join the organisation at the beginning of his career as a champion was tantamount to declaring there was nothing, apart from political power, that white America had that he wanted; it said he was not about to be bought off. "We weren't about to join the Nation," African-American writer Jill Nelson told Ali biographer David Remnick. "But we loved Ali for the supreme act of defiance. It was the defiance against having to be the good Negro, the good Christian waiting to be rewarded by the righteous white provider."

If America was divided from within, it was also felt under threat from outside. Just a year before Ali had mounted the Olympic podium in Rome, Fidel Castro had arrived in Havana at the head of the Cuban revolution. Just a year later, the Soviet Union sent the first man into space. The racial mayhem in the South was a severe embarrassment to a country preaching the virtues of human rights and democracy to the Eastern bloc. Just emerging from McCarthyism, and with anti-communist firebrand J Edgar Hoover at the helm of its secret police, the American psyche was as fragile as its military was powerful.

This contradiction was being played out principally in Vietnam, where US attempts to defeat the Vietcong were becoming increasingly desperate and vicious. By the end of the 1960s, Pentagon papers reveal, the nation's war aims were "70% to avoid a humiliating US defeat" and only "10% to permit the people of South Vietnam to enjoy a better way of life".

A li was offered an easy option if he would agree to be drafted: he could fight exhibition matches, as Louis had done during the second world war, to entertain the troops. So when they called out his name in the induction centre in Houston and he refused to step forward, it was a purely moral stand.

Within an hour he had been stripped of his title and had his boxing licence suspended. Later he was sentenced to five years in prison and fined $10,000. It was the beginning of a three year exile from the ring. He would have to wait seven years to get the title back. By this time, 1967, he was used to the vitriol that was poured on him from all sides. "Cassius makes himself as sorry a spectacle as those unwashed punks who picket and demonstrate against the war," wrote Red Smith of the New York Herald Tribune.

One of the most striking depictions of the latest biopic is the intense, private loneliness Ali felt during this time. His primary source of earnings and means of asserting himself over his environment was boxing. With that taken away he was left with his conscience and renown - great for his moral fibre but not much to feed a young family or with which to build a career for his future.

Paradoxically, it was in his most isolated period that he touched the consciousness of so many worldwide. Transcending race, nationality and creed, his statement: "I ain't got no quarrel with them Vietcong," would reverberate through the peace movement in the west, through the African-American community and through the increasingly independent countries of the developing world.

"It's hard to relay the emotion of that time," says the poet and activist Sonia Sanchez. "Imagine it for a moment! The heavyweight champion, a magical man, taking his fight out of the ring and into the arena of politics and standing firm. The message that sent."

The message crossed the Atlantic. "We knew Muhammad Ali as a boxer but more importantly for his political stance," says artist, Malick Bowens in the film When We Were Kings. "When we saw that America was at war with a third world country in Vietnam and one of the children of the US said, 'Me? You want me to fight against Vietcong?' It was extraordinary that in America someone could have taken such a position at that time. He may have lost his title. He may have lost millions of dollars. But that's where he gained the esteem of millions of Africans."

In Armistead Maupin's novel Maybe the Moon, the protagonist describes what it is like to be a person of restricted growth. "When you're my size and not being tormented by elevator buttons, water fountains and ATMs you spend your life accommodating the sensibilities of 'normal people'," she says. "You learn to bury your own feelings and honour theirs in the hope that they'll meet you halfway. It becomes your job, and yours alone, to explain, to ignore, to forgive - over and over again. There's no way you can get around this. You do it if you want to have a life and not spend it being corroded by your own anger. You do it if you want to belong to the human race."

Most people who have had to negotiate not just the most obvious manifestations of discrimination, but also the petty, daily, social expressions of prejudice which serve to undermine and overwhelm can identify with this. What Ali did was help shift Maupin's "halfway" point away from the powerful and towards the powerless. His own personal reinvention demanded, of everyone, not only a reassessment of your relationship to him, but to the issues he raised. He was the champion and to some extent he could dictate the terms of engagement.

Years later, Ali said: "I liked being who I was because they would put me on television and when I say, 'I'm the greatest, I'm pretty', that means that little black children and people who felt like nothing say, 'We got a champion. Look what he's doing. Look at him over there.' "

I was one of those children. At home we had a rabbit - a black rabbit with a white patch around its eye - that we called Muhammad. And I had a T-shirt, with wording stretching over my toddler's pot belly reading: "Float like a butterfly, sting like a bee." I never thought I was nothing. But when I wore that T-shirt, I felt like a million dollars.

Ali is released on February 15, as is the documentary Muhammad Ali: the Greatest.