To return from the United States to Europe is to travel from a country that thinks it is on the front line of the struggle against jihadist terrorism but is not, to a continent that is on the front line but still has not fully awoken to the fact.

Only a fool would rule out the possibility of another terrorist assault on what is now styled the American homeland, but the fact is that in the six years since 9/11, there have been several major attacks (Madrid, London) and foiled plots in Europe. In the United States, there have been no major attacks and, as far as we know, just a few averted conspiracies. All the evidence suggests that American Muslims are better integrated than those in Western Europe. Last week’s arrest of a group apparently planning a 9/11 anniversary attack in Germany suggests that the threat to the heimat is greater than that to the U.S. homeland.

An invisible front line runs through the quiet streets of many a European city or town where there is a significant Muslim population. Whether you live in London or Oxford, Berlin or Neu-Ulm, Madrid or Rotterdam, you are on that front line -- much more than you ever were during the Cold War. This struggle is partly about intelligence and police work to prevent those who have already become fanatical, violent jihadists from blowing us up at St. Pancras or the Gare du Nord. Ordinary non-Muslim Europeans can only do a little to help this work, as well as worrying about the curtailment of civil liberties. Ordinary, peaceful, law-abiding Muslim Europeans can do a little more.

The larger part of this struggle, and the more important in the longer term, is the battle for the hearts and minds of young European Muslims -- usually men -- who are not yet violent jihadists but could become so. All over the Continent, and around its edges, there are hundreds of thousands of young Muslim men who could be tomorrow’s bombers -- or tomorrow’s good citizens.

The chemistry in Europe can be understood a little better by thinking back to the last wave of youth terrorism, in the “German autumn” of 30 years ago and Italy’s Red Brigades. When I lived in Berlin in the late 1970s, I met quite a few people who told me, “You know, there was a moment when I could have gone either way.” They could have slunk away to join the Red Army Faction, like those acquaintances of their acquaintances. Instead, they became journalists, academics or lawyers and are now pillars of a society under attack from a potentially more destructive wave of terrorism.

Of course, we cannot take the comparison too far, but one basic feature is the same: Beside the hard core of fanatics is a penumbra of people who couldchoose the wrong path. In Germany, they are called the sympathisanten -- the sympathizers. Among European Muslims, they might very roughly be correlated with those who, in surveys, refuse to condemn suicide bombings. One analyst estimates that while the hard core may make up 1% of British Muslims, the sympathisanten make up perhaps 10% of German Muslims.

If you look at the biographies of the jihadist assassins over the last six years, from the 9/11 bomber Mohamed Atta, radicalized in Hamburg, to Mohammed Bouyeri, murderer of Theo van Gogh, you find again and again the same story: young men who were first attracted to a modern, Western way of life, quite different from that of their parents, but who then angrily rejected it in favor of a violent, extremist version of political Islam.

Fortunately, there are also people who travel the other way. So much now depends on whether the 10% veer toward the barbaric 1% or rejoin the civilized majority.

Iraq is a sideshow in this larger struggle. President Bush may claim that Iraq is the front line in the war on terror, but even some of his senior commanders don’t believe that. To be sure, the Iraq war has become an added grievance for disaffected Muslims everywhere, although note that Germany’s nonparticipation in the Iraq war did not keep it safe. Nor should we avert our eyes from the further uncomfortable truth that a U.S. withdrawal from Iraq will be celebrated by violent jihadists as a victory.

But the larger truth is that a British soldier returning from Basra to Bradford (a city with a large Muslim population) will be coming from one front line to another. This invisible front line is not a military but a cultural/political one. The returning soldier may do more to reduce the threat of terrorism in Britain by his off-duty attitude toward British Muslims in his hometown than by anything he did, gun in hand, in Basra.

Afghanistan is a different matter. Rooting out the original Al Qaeda and beating back the renascent Taliban is an integral part of combating jihadist terrorism. So is trying to change the poisonous mixture of radical religion and politics in Pakistan and Saudi Arabia. The man who seems to have been a ringleader of the German group, a convert to Islam called Fritz Gelowicz, was radicalized in an Islamic center, the Multi-Kultur-Haus (another blow to the good name of multiculturalism), in Neu-Ulm by instructors from the toxic Wahhabi sect of Islam, based in and funded by that great American ally, Saudi Arabia. He then reportedly went for Arabic-language training in Syria and terrorist training in the border regions of Pakistan. According to German sources, the instruction to launch the anniversary attack came by e-mail from Pakistan. So death can come out of Neu-Ulm by way of Waziristan.

If we are calm, clear-sighted and resolute, we will eventually win this struggle and remain free. A continent that has rid itself of the horrors of imperialism, fascism and communism will see off this lesser menace too. But it will take many years, and we had better shape up to it.

Timothy Garton Ash, a contributing editor to Opinion, is professor of European studies at Oxford University and a senior fellow at the Hoover Institution at Stanford University.