Timothy Garton Ash: A moment that will define the 21st century

'Will it be like the Kennedy assassination - shocking, unforgettable, but ultimately of little import for the course of history?'

Where were you when you heard the news that Kennedy had been shot? That the Berlin Wall was coming down? And now, that the World Trade Centre was under attack? This was one of those defining moments of global experience and emotion, shared through television. You walk down the street, and you know that everyone around you is thinking of the same thing. And hundreds of millions with them, across the world.

But the big question is: which kind of global event will it be? Will it be like the Kennedy assassination – shocking, unforgettable, but ultimately of little import for the course of history? Or will it be more like the fall of the Wall, an event that really does change the course of history, with consequences that are played out over many decades and all the continents?

My hunch is that it will prove to be somewhere inbetween, but closer to the latter – for two reasons. First, because this was a catastrophe foretold.

For years now, security experts have been warning us that, after the end of the Cold War, the greatest threat to the security of our prosperous capitalist democracies – the West or the North, according to your point of view – might come from terrorist attack. Most people did not quite believe it. Yes, there have been horrific bombings, but there has been no really large defining moment – no Berlin blockade or Cuban missile crisis of the new age – to impress it upon every mind. Well, here it is: images of the most famous cityscape in the world wreathed in smoke and changed utterly. So, far from being a one-off freak occurrence, this is the worst-case realisation of deeper trends already charted and foreseen.

Second, I think that it will change the course of history because what happens in the world at the start of the 21st century depends more than ever before on the conduct of a single country, the United States, and this attack seems likely to have an incalculable impact on the pyschology of that country. So much of the great and largely benign continuity of American foreign policy since 1945 has depended on the outside world not impacting directly on the lives of most Americans.

Anyone who has spent time in America will know what I mean. People grumbled in smalltown bars about entangling alliances. Congressmen and commentators in Washington made threatening noises about isolation or retribution. But most of the people, most of the time, really didn't care that much what happened in the rest of the world, one way or the other.

On that solid foundation of deep popular indifference the soaring steel towers of American foreign policy were erected by élite architects. With the most significant external attack on the heart of the American homeland since British forces burned Washington in 1814, that paradoxically solid foundation will be shaken. It may seem odd to fear the moment when ordinary Americans really start to care about the outside world, but we may yet find ourselves longing nostalgically for the old, self-contained indifference that has so often annoyed the foreign visitor.

There are many things that public opinion has prevented American leaders doing in the world. Since Vietnam, for example, there has been the phobia about risking the lives of American soldiers for fear that they would come back in bodybags.

Hence the bombing of Kosovo from a safe altitude of 15,000 feet. But there were very few things that public opinion actively compelled leaders to do in foreign policy. The Manhattan horror seems to have changed that – for the time being, anyway. Suddenly, the cry goes up from throats half-choked with debris and dust, and from millions across America: revenge! Get the bastards who did this, preferably with smart weapons and no casualties, of course, but if not, if it means some bodybags – well, so be it.

So what happens now, in the post- 11 September world? Here, to concentrate the mind, are three scenarios:

Scenario One. The United States starts behaving more like Israel. Feeling itself embattled and besieged, but with a manifest destiny, it lashes out with its hi-tech military at anyone who might even seem to want to attack it. Any terrorist attack provokes instant retaliation, without waiting for proof that the attack actually came from that source. An eye for an eye, a tooth for a tooth – and never mind exactly whose eye or tooth it is. This is the course being urged on America by Ehud Barak, the former Israeli prime minister. America, he says, should go to war against terrorism – all known terrorists.

One should never underestimate the influence, not so much of Israel directly, but of the example of Israel on the Republican right in the US. In the 1980s, for example, there were curious but strong connections between the ruthlessness that Israel was showing in Lebanon and the toughness that the Reagan administration chose to demonstrate in Central America.

Many of the first reactions in Washington seem to point in this direction. Colin Powell, the US Secretary of State, says that, whatever the legal position, most Americans feel their country is at war – and he feels that way, too. Everyone talks of Pearl Harbor, and of swift and certain retribution. Against whom? "I have no doubt in my mind that it's Osama bin Laden," says Senator John Kerry, of Massachusetts. Now, Osama bin Laden, the Saudi billionaire terrorist, is protected in Afghanistan by the ruling Taliban. And President Bush himself says that "we will make no distinction between the terrorists who committed these acts and those who harbor them".

So: bombs away over Afghanistan, innocents are killed along with the guilty, and more waves of anger roll towards the United States from parts of the Arab and Islamic worlds? The US as Greater Israel.

Scenario Two. The West versus the rest. With countries like Britain standing "shoulder to shoulder" with the United States, as Tony Blair has promised, the Bush administration takes a more considered approach. Instead of America wreaking unilateral vengeance, a strategy is put together in conjunction with America's Western allies. But the coalition does not extend much beyond the Nato allies and a few other traditional friends of the West.

This larger West itself becomes embattled. Britain finds itself in the front line, with British landmarks such as Canary Wharf enduring the same fierce security as will doubtless now apply to the office towers of Manhattan. There is, for years to come, an ongoing battle with the diverse and constantly shifting forces of terrorism. The terrorists find shelter in states that we describe as "rogue", but which see themselves as brothers in Islam, brothers in anti-Zionism or simply brothers in the great alignment of the world's poor against the world's rich. They in turn are tacitly supported by greater powers, such as China, that seek allies or clients in their own global game.

Scenario Three. The United Nations against the terrorists. Displaying the patience and restraint that he showed in handling the crisis earlier this year when an American spyplane was brought down in China, President Bush allows the time needed to establish with a reasonable degree of probability who was actually responsible for these attacks. Direct American armed reprisals are limited to them. At the same time, he works with and through the UN to establish a coalition for action against terrorism that is wider than the West. In particular, it includes Russia and China. At moments, it has seemed as if the Bush administration has seen the world entering into a new version of the Cold War, with China in the role of the new Soviet Union. But it is not China that has struck at the heart of America.

Such painfully co-ordinated international action may be less effective at stopping particular terrorists in the short term, but its longer-term effect is to pull together disparate states with the most powerful glue of all: a common enemy. Instead of Samuel Huntington's "Clash of Civilisations", there is the defence of civilisation – in the singular. And the bedrock of civilisation includes the human rights of all, and international law applied equally to all.

These three scenarios start from the immediate response to what President Bush accurately described as "mass murder". But the implications stretch far beyond.Ever since George Bush was elected, we have been speculating how far he is prepared for the United States to "go it alone". In the jargon: will he be unilateralist or still multilateralist? Now, in the most extreme circumstances, we shall find out.

It may seem wild to suggest that how the US responds to a terrorist attack, however large and horrific that attack, will shape the whole international system. It may yet be true. If the fall of the Berlin Wall was the true end of the short 20th century, there is a good case for arguing that the demolition of the World Trade Centre was the true beginning of the 21st. Welcome to another brave new world.

The writer is a fellow of St Antony's College, Oxford