Timothy Garton Ash: We know what Europe is for, but what is it against these days?

'It's more difficult to define yourself against America when we seem to be under attack as one civilisation'

Identities are defined not just by what you are for and who you are with, but above all by who or what you are against, or what you feel is against you. This is often an outright enemy, but it may just be a great rival – the other team, so to speak. In the jargon of identity studies, it is the Other. The deepest question posed to Europe by this continuing "war against terrorism" is: who or what is Europe's Other?

During the Cold War, the answer was plain. Europe's Other was the threat from the Communist "East". There were other Others, too: Europe's own bloody past was a kind of historical Other, the United States a very important rival for Gaullists of all countries. But this was the main one.

Since the end of the Cold War, Europe has been a continent in search of its Other. In the last decade, many on the European left saw the Other in the United States. Europe was to be defined as the not-America. Europe would preserve a different and more "social" model of democratic capitalism, even in an era of globalisation. Europe would be a counterweight to the crude, brash, only-surviving-superpower, with its misguided policy in the Middle East, its lamentable record on aid to the Third World, and a general tendency to throw its weight around.

This view has not simply disappeared after 11 September. Indeed, there has been a lot of criticism of the US during the war, and many Europeans argue that 11 September shows the need for a more sophisticated, multilateral approach to a complex, naughty world. But it is more difficult to define yourself primarily against America at a time when America and Europe seem to be under attack, as part of one Western, Christian or post-Christian, materialist, decadent civilisation. In that attack, Osama bin Laden thrusts upon Europe the prospect of another Other, at once very new and the oldest of them all. For "Europe" was originally defined as a conscious entity in the conflict with the Islamic world. The first political usage of the term comes in the eighth and ninth centuries, as the followers of the Prophet – the "infidels", in Christian parlance – are thrusting, by force of arms linked to a faith that we would now call fanatical, into the underbelly of Europe. "Europe" begins its continuous history as a political concept in the 14th and 15th centuries, first as synonym for, then as successor to, the Crusaders' notion of Christendom – and once again, its Other is plainly the Arab-Islamic world.

There is a real temptation to revive that ancient European bogey. One European leader has spectacularly succumbed to the temptation. The Italian prime minister, Silvio Berlusconi, told journalists we should have confidence in the superiority of our culture. "The West," he said, "given the superiority of its values, is bound to occidentalise and conquer people. It has done it with the Communist world and part of the Islamic world, but unfortunately a part of the Islamic world is 1,400 years behind." Mr Berlusconi made the remarks after a breakfast with Vladimir Putin. In a volcanic essay, thejournalist Oriana Fallaci added "We might as well admit it. Our churches and cathedrals are more beautiful than their mosques.' And she described Arab immigration to Italy as "a secret invasion"'. Is it an accident that these two voices come from Rome, the centre of Western Christendom?

However, this is not just about Western Christendom. President Putin's remarkable strategic response to 11 September, immediately and wholeheartedly positioning Russia with Europe and the West, is justified ideologically by an implicit claim that the world of Eastern Christendom, of Orthodoxy, stands on the front line against Islamic and "Asiatic" barbarism (typified for Mr Putin by Chechen and Afghan "terrorists").

Most European leaders and intellectuals of course reject this polemical (re)construction of our identity. Even if some claim of cultural superiority were justified – and the record of European barbarism in the 20th century should make us humble in that regard – it would be madness for Europe to embrace this rhetoric. The whole West is already at risk of alienating Muslims throughout the world in what George Bush once ill-advisedly called our "crusade".

This would be particularly dangerous for Europe, which sits just a few miles north and west of a diverse, frustrated, and in large parts impoverished Islamic and Arab world, in what Europeans used to call the Near East, in North Africa, the Caucasus and Central Asia. Above all, it would be suicidal for a continent in which as many as 20 million Muslims already live.

Although London and a few other English cities have their share of Islamic radicals, most British Muslims seem reasonably at home in British society. It is important that we should help them become more so. The Turkish communities in Germany are less well integrated. A senior German politician said Germany had more extremist teachers of Islam than Turkey did.

And a few weeks ago, in a working-class quarter of Madrid, I spoke to a illegal immigrant, aged 23, from Morocco, called Yacine. Yacine came to Spain hidden under a bus. He does not have the papers to get a job, so he lives by stealing. Did he think the Western response to 11 September was directed against Islam? "Yes, it's an attack on Islam." Many of his relatives, he added, "think the Jews will have a part in the attack – and so do I."

Muslims in Europe will not be reassured simply by President Bush or Tony Blair pronouncing, as fresh-baked Koranic scholars, that Osama bin Laden's message is a perversion of Islam. As the French writer Olivier Roy has argued, we need a much deeper reflection on what it means to talk of European Muslims or "European Islam". The very notion challenges deep assumptions about Europe as post-Christendom that one often glimpses beneath the elevated rhetoric of European unification. We must therefore hope that this latest new-old Other is immediately put back in its box, and the lid firmly closed – although some Muslims will suspect that Mr Berlusconi was merely saying what Europeans really think. Meanwhile, the Russian Other is largely gone, especially if Mr Mr putin continues his pro-Western course. The American Other remains a candidate, but one that looks rather out of place in the post-11 September world. In the end, it will never fit the bill, for these are not, in fact, two separate civilisations, but one, albeit containing a wide spectrum of social, economic, and political models, ranging from the American Right to the French Left. And there is no other Other in sight.

Thus the task for those who believe, as I do, in a project called "Europe", is to build a strong, positive European identity, one that binds people emotionally to a set of institutions, without the help of a clear and present Other. The "war against terrorism" clarifies that task, but also complicates it. For the time being, I must conclude that this is yet another defining moment at which Europe declines to be defined.

The writer is a fellow of St Antony's College, Oxford. A longer version of this article appears in the current 'New York Review of Books'