It was May 29th 1968—towards the end of what was probably the most dramatic month of certainly the most dramatic year of the second half of the 20th century.

There was a workers' march in Paris, by no means the first that May. There was no hint of violence or confrontation, which meant the protest barely constituted news at all. The original student rioting had mutated into a general strike, and the police had more or less withdrawn-along with President Charles de Gaulle, whose whereabouts were unknown. There was an eerie calm.

"I had the feeling", recalled Edward Mortimer, then a young Paris correspondent for the Times, "that had they just turned off the route and gone in to the courtyard of the Elysée Palace, no one would have stopped them, and that would have been the end of it."

"It" meant de Gaulle's decade of almost absolute power, the Fifth Republic he created, and-perhaps—the entire political structure of the Western world, then besieged by a host of different clamouring enemies. Indeed, Mortimer's erudite colleague, Patrick Brogan, wrote an obituary for the Fifth Republic ("...its end matches its beginning") for the following day's paper. That was the day de Gaulle reappeared and started a spectacular fightback. His disappearance had been to Germany, to consult secretly with his military commanders and test the loyalty of the troops. The Fifth Republic lives yet.

Any history of 1968 is full of if onlys, what ifs, counterfactuals and might-have-beens. You can construct a zillion parallel universes of what could have happened if a single event had turned out a fraction differently. The reality was astonishing enough. And, as the 40th anniversary approaches, the year continues to maintain a remarkable grip on anyone with a sense of modern history.

There was quite simply an eruption: a global explosion of political, cultural and social energy that was at once both thrilling and terrifying. It had been rumbling and spitting for at least five years beforehand (since 1963, the year Philip Larkin said sexual intercourse was invented) and continued to send out lava flows for some time after that. The activist and writer Tariq Ali dates the real end of '68 as the Portuguese revolution of 1974.

There was also a short version of '68—that brief period when, as on that Paris march, almost anything seemed possible: when "the revolution" (whatever that might have meant), as discussed in a thousand French cafés, and dingy student dormitories and flats across the world, felt like an imminent certainty.

Perhaps that even ended on May 30th, when de Gaulle regained control of France. It was certainly over by the end of summer, when reformers were crushed in places as disparate as Prague and Chicago. By December, enough calm had returned for Time magazine to name as its men of the year the three American astronauts who spent Christmas escaping the increasingly surly bonds of Earth to orbit (but not land on) the moon in Apollo 8: Borman, Lovell and Anders. That did seem to miss the point somehow.

But what was the point? All year, there was a seemingly endless supply of startling global headlines. However, it is still hard to discern a coherent picture in the mosaic.

The unifying factor is that across the developed world and even beyond, people—specifically the young and most specifically students-challenged established authority. Sometimes they did this violently and sometimes this invited violent consequences. Inevitably, American concerns linked in to everyone else's. The United States was obsessed with the intractable war in Vietnam. And there was a very obvious tipping point.

On January 30th nearly 70,000 Communist fighters (North Vietnamese soldiers and Viet Cong guerrillas) broke the truce agreed on for the Vietnamese New Year—Tet—and surged out of the countryside into the cities, including the seemingly immune capital of American-backed South Vietnam, Saigon. Their most spectacular feat was a briefly successful commando raid on the American embassy.

Until this point, the guiding narrative had been provided by American officialdom. President Lyndon Johnson believed the war was necessary and winnable; his commander in Vietnam, General William Westmoreland, constantly assured him and the folks back home it was being won. Now even Johnson began to doubt.

In February Walter Cronkite, the avuncular anchorman of CBS News, went to Vietnam to see for himself. His reporting was sceptical, perplexed; he could see no victory. "If I've lost Cronkite," Johnson reportedly remarked, "I've lost middle America."

Tet changed everything. The military's response was to demand a surge: 206,000 extra troops in addition to the 550,000 already committed to Vietnam, a fact that hit the New York Times on March 10th, two days before the New Hampshire Democratic primary, which was assumed to be a formality for Johnson. There was an anti-war opponent but nobody took him seriously. He was the senator from Minnesota: a self-effacing, sardonically witty ex-college professor called Eugene McCarthy (who was no relation whatever to the 1950s Red-baiter, Joe).

Almost beneath the notice of the media, hundreds of students had travelled to New Hampshire to work for McCarthy. Those who looked over-hippyish, in the style of the time, were ordered to cut their hair and shave ("Clean for Gene") or confined to back-office duty. The rest took to the streets.

In mid-February polls showed McCarthy getting 20% of the vote. He got 42%. Four days later, on March 16th, Bobby Kennedy, brother of the martyred president, also entered the race as an anti-war candidate. That was the day—though no one in the United States would have then known it—when American troops killed hundreds of villagers at My Lai, a sign that the soldiery had become dehumanised to the point of barbarism. Two weeks later, on March 31st, Johnson stunned even his closest advisers by withdrawing from the election campaign and initiating peace talks. Five days after that, Martin Luther King was murdered in Memphis, causing a fresh outbreak of angry black rioting.

For young Americans, so privileged in many respects, the campaign against the war was not some theoretical cause, any more than the black demands for civil rights were. They were subject to conscription, and liable to be shipped out to Vietnam where their contemporaries were dying (at a rate of 300 a week in 1968, compared with nearly 4,000 in all so far in Iraq). It was easy enough to gain serial deferments (Bill Clinton, Dick Cheney) or a cushy number back home (George Bush junior) if you were smart enough or influential enough. But just about every American community now had someone who had been killed or maimed.

Elsewhere in the West the grievances were less obvious. Even the British government had the wit to keep its troops out of Vietnam, despite strong pressure from Johnson on the prime minister, Harold Wilson. But opposition to the war was global. And campus rebellion spread globally, sucking up local issues along the way—maybe only Maoist China, then more remote than the moon, marched to a totally different drummer.

It was partly imitative: there had been demos and sit-ins on American campuses in 1967-and American style, as ever, led the way. There were shared icons (notably Che Guevara, the guerrilla leader recently killed in Bolivia, and the largely unread sociologist Herbert Marcuse, author of the anti-capitalist tract "Repressive Tolerance") and a hotchpotch of grievances. A global investigation in the Times headed "Students in Revolt" highlighted "the war in Vietnam, racial discrimination, consensus politics and overstrained educational systems".

"The post-war economic boom had led to the phenomenal expansion of higher education all over the world," recalls Tariq Ali, then editor of the newspaper Black Dwarf and bête noir of the British establishment. "Yet you still had pretty rigid sexual and social codes."

Sex was (naturally) a particularly strong catalyst in France: the unrest started at the bleak and overcrowded suburban campus at Nanterre, where protesters were particularly cross about stern rules concerning male visits to women's dormitories.

The often cited casus belli was a bizarre confrontation between a student, Daniel Cohn-Bendit, and the Gaullist youth minister, François Missoffe, who had come to Nanterre to open a swimming pool. Cohn-Bendit complained that nowhere in the ministry's long report on youth was there a mention of sexual problems. "No wonder, with a face like yours, you have these problems: I suggest you take a dip in the pool."

Not since the French sent Henry V tennis balls and precipitated Agincourt has an insult caused more trouble. Within months Cohn-Bendit—red-haired, articulate and very cross—was "Dany the Red" and world-famous (and presumably not short of women). The crisis grew exponentially. The government scented a worldwide conspiracy. "It is not possible that all of these movements could be unleashed at the same time, in so many different countries, without orchestration," said de Gaulle. He over-estimated his enemy. The Times concluded that there was no conspiracy, merely mutual inspiration and a certain amount of cross-pollination. The mass communications of 1968 (no internet, no fax, a rudimentary international phone system) would seem primeval in 2008 but TV pictures via satellite meant that news travelled at a pace unknown even a decade earlier. It could now pierce the iron curtain. Though a student revolt in Poland was put down with police clubs, in Czechoslovakia the stars were momentarily in alignment: the youthful mood coincided with the arrival in power of the reform-minded Alexander Dubcek.

However, in a less globalised world than today, the local did tend to trump the generic. The Vietnam peace talks began in Paris at the height of the French disturbances: Edward Mortimer happened to mention this to one of the demonstrators. "Yes, yes," he replied distractedly. "But haven't you heard? There are barricades in Clermont-Ferrand!"

It is a little hard to equate Parisian kids' demands for easier sex with the carnage that had scarred Vietnam or communist repression, but there was a link. "Students in the West grew their hair long or whatever to show they weren't capitalists or warmongers like their parents," says historian and author Mary Heimann. "Students in Czechoslovakia grew their hair long and wore jeans to show they weren't rigid old-fashioned communists."

"There was something vague floating in the air at the time, like a tempest," recalls the philosopher Daniel Bensaïd, one of the student leaders at Nanterre. "It wasn't a plot, it wasn't a coincidence, it was a moment in history."

And then it passed. On May 30th de Gaulle re-appeared, made a dramatic radio address and called a parliamentary election. There was a sharp swing to the right. "It's the paradox of '68," muses Bensaïd now, "the biggest revolt, the biggest rightist majority."

Perhaps it wasn't such a paradox. On the streets of Paris, it was often exhilarating. And despite the massive numbers, only a handful were killed in the whole insurrection. That's not how it looked on the TV screens of la France profonde: voters took fright.

As soon as France went quiet, Bobby Kennedy was shot dead by a lone Palestinian gunman, Sirhan Sirhan, moments after delivering an eloquent victory speech in the California primary. There was shock but not surprise. "They shot him," people were heard to cry in the confused aftermath. In the atmosphere of the time, no one had to ask who "they" were: "they" constituted the sinister forces of established authority out to get anyone who threatened their power. With JFK shot, and mlk, and now Bobby, it looked as though any challenger was a target: "They are going to kill off all our leaders," said one activist. No matter that Sirhan was an embittered Palestinian, and Bobby was American royalty. "They" had prevailed.

Kennedy's murder ensured that "they" would provide the next American president: either the vice-president, Hubert Humphrey, or the former vice-president, Richard Nixon. The brief shining moment of McCarthy's New Hampshire triumph had been subsumed in the bloody civil war for the soul of the left between his supporters and Bobby's.

That rancour did not die until McCarthy did, aged 89, in 2005. I interviewed him in 2003, when he was in a Washington old people's home, and largely forgotten; what shone through was his enduring wit (I found him reading "Lord of the Flies": "It's about the Bush administration, you know," he said)—and his bitterness about the Kennedys.

His campaign had always had a strange disengaged quality: Jeremy Larner, McCarthy's speechwriter, said he never once heard him discuss what he might do as president. And that grew more pronounced. "Bobby's assassination killed Gene too in a way," Larner recalls, "He really didn't want to campaign at all after Bobby was shot."

Under the rules in place at the time (abolished for 1972), it was possible to win the nomination without taking a single primary by stitching up the party machine, which Humphrey was doing. Larner argues, convincingly, that Kennedy's charisma and money could have bust this wide open. Without him, there was no chance. The Chicago convention was a farce and, on the streets where police were beating protesters, a bloody disgrace.

That same week, the Soviet Union moved its tanks into Prague and asserted control over the government. "Socialism with a human face" was over then and for ever. Moscow believed that Dubcek was incapable of controlling the forces that had been unleashed, and that the country might tilt towards the West. Moscow was right. Communist control of eastern Europe was halfway through its baleful reign. No one again believed it could be made compatible with liberty. The Vietnam peace talks stalled—many believe they were sabotaged by an alliance between President Thieu of South Vietnam and American presidential aspirant Nixon. For both, success in Paris would have been a disaster.

And so, gradually, the Earth resumed its former orbit. Perhaps it hadn't really been away. Britain was only a bit player in the political upheavals, but culturally it had never been more influential: foreigners, anyway, believed its capital was "Swinging London". The most durable number-one single of 1968 in Britain (in an era when teenagers bought 45rpm vinyl discs as frequently as they bought cigarettes) was "Those Were The Days" by the twee Welsh chanteuse, Mary Hopkin.

The leaders of rock culture kept pushing the boundaries of music, drug use and onstage outrage. In 1968 the Who were best known for destroying their instruments; but they were quietly working on the rock opera "Tommy". The Beatles were no longer cuddly moptops, and most of their headlines were bad ones. But "Sergeant Pepper" was still number-one album at the start of the year (vying with "The Sound of Music"), and the White Album came in with the lunarnauts. But the new had not displaced the old at all. Dominic Sandbrook ends his history of 1960s Britain, "White Heat", with a dissertation on the TV comedy hit of 1968, the wartime wallow "Dad's Army", and notes: "Although the Sixties is often seen as a period of Utopian optimism, the culture of the time, from the albums of the Beatles and the Kinks to the poems of Philip Larkin and the novels of John Fowles, was suffused with a powerful sense of nostalgia."

These disjunctions spread into politics. Bill (now Lord) Rodgers, one of the last 1968 government ministers still alive, confirms that Wilson was far more worried about a right-wing Labour plot to install the chancellor, Roy Jenkins, in his place than he ever was about student revolution. The major British political event of the year was the anti-immigrant "Rivers of Blood" speech by the Tory politician Enoch Powell. Polls showed that Powell, unanimously condemned at the court of elite opinion, had overwhelming public support. All year, there were massive swings to the Conservative opposition in by-elections, suggesting nostalgia even extended to the rule of Sir Alec Douglas-Home.

This was not just a British phenomenon. There was also the French election, and Nixon's capture of the American presidency. Indeed, research suggested that many of those who had voted for McCarthy in the New Hampshire primary later voted not for Humphrey, nor for Nixon, but the overtly racist third-party candidate, George Wallace.

In 1969 Britain gave 18- to 21-year-olds the vote as a sop to generational demand; when the nation indeed returned the Tories at the 1970 election, the new voters marched in step with their elders. On campuses, the protests became more muted, and all the echoes with them. By the early 1970s, Western assumptions of endless prosperity began to look more questionable—the next generation of students would have sterner priorities. The tumult and the shouting died away. So what did it all achieve? What happened to it all?

"What happened to it all? what happened to it all?" Felix Dennis was roaring now. "We changed the fucking world, that's what, you scumbag."

Dennis, connected with the radical magazine Oz in 1968 and later a publishing tycoon in his own very capitalist right, grew a little more respectful once he discovered I was old enough to have my own memories of the year. He remained pretty vehement.

The world did change. But who changed it? It depends on what you mean by "we". Stuck that year in a boarding school in the English countryside, I didn't change much, though I think we successfully agitated for a liberalisation of the rules on leaving the premises.

And out in the big wide world the great legislative reforms of the 1960s came not from the young agitators but the older generation: it was the reviled Johnson who pushed civil rights legislation through a still-reluctant Congress. Anti- homosexuality laws started being repealed across America from 1962; Britain followed five years later.

What didn't change at first was attitude. Casual racism had certainly not disappeared. And the old cultural taboos against homosexuals remained in place: there were no openly gay mainstream pop stars until the late 1970s and, outside the still secretive subculture, the word "gay" continued to mean blithely happy.

Furthermore, the causes—more coherent and achievable than "revolution"—that would eventually flow from the 1960s had not yet shown themselves. The term "women's lib" gained common currency only in 1970, and even then was used by most men with a wry smile. Paul Ehrlich's jeremiad "The Population Bomb" was published in 1968, but environmentalism also did not become fashionable until the next decade before fading again, along with Ehrlich's predictions.

Yet Dennis is not wrong. You can see 1968 as the hinge on which the post-war world hangs; and the screw in that hinge was the screwing. "What everyone forgets is that the pill had just arrived," says Dennis. "There was no aids, no herpes. Syphilis was in absolute retreat and even gonorrhea was very rare. You might get a dose of the crabs. There was free sex with no downside. It was a very small window and it lasted about seven years.

"And women were walking down the street in mini-skirts, in what looked like their underwear. It was almost too much for anyone to stand."

Even so, the sexual implications of the fashion were still publicly discussed only in code. British reticence, American Puritanism, Continental religiosity and Communist repression saw to that. The change towards open discussion developed only very slowly.

The same applies to related social developments: in 1968 most young men still wore pyjamas and white y-fronts, even if they were now readier to take them off; casual drug use slipped only slowly from the avant-garde to the high-school playground; and very long hair for men did not become the norm until the 1970s (all the really serious student revolutionaries I knew in that era kept their hair determinedly short). Social change was far slower than history or selective memory might suggest. But 1968 made it unstoppable.

"In the end, it was a sensational victory," says Felix Dennis, "the hidden victory that set up the 21st century. And on that, the whole Western world floats as it does."