Mr. Laqueur, a historian, is the author, inter alia, of "A History of Terrorism" (Little Brown, 1977).

Whenever a major terrorist attack occurs, the politicians' speeches are predictable -- that the terrorists are barbarians, that they won't succeed in destroying our values and undermining our way of life. All this is correct and probably bears repeating, even though it's not exactly new and doesn't help us understand what happened and what may happen in the future.

Also predictable at these times are the voices that admonish us to make a greater effort to understand the motives of the people who perpetrate these outrages and to enter a dialogue with them. In the Frankfurter Allgemeine Zeitung, one commentator admonished us to understand that many people admire the terrorists -- which is perfectly correct, but not really a guide to action for the same can be said with regard to the late Adolf Hitler and Josef Stalin. The Guardian last week said that the war against terrorism will not be won until we better understand the Arabs and their grievances.

But the war against terrorism will not be won in our time in any case. Terrorism is the contemporary form of violent conflict, as major wars have become too costly and conflict won't disappear from the face of the earth in the foreseeable future. As for the Arabs and their grievances, the search for last Thursday's London attackers lead in all kinds of directions, with the early speculation being that North Africans and Pakistanis (of British birth) were behind it rather than Arabs from further east.

The politicians also tell us that the great majority of Muslims have nothing to do with terrorism, which is perfectly true. But it is also true that many thousands of young men in these communities in Britain and France, in Spain and Italy, sympathize with the jihadis. Some of them know where the terrorists are hiding but won't tell the authorities. Terrorists cannot exist in a vacuum; they need a periphery of helpers. That's the main problem now facing the security services. But politicians are reluctant to press the point by strongly admonishing Europe's Muslims to do their civic duty and cooperate in finding the terrorists.

According to the security authorities, between 600 and 3,000 people in Britain graduated from Osama bin Laden's training camps in Afghanistan and could launch further attacks any day. (The higher figure is probably exaggerated.) But these people have been around for a long time and few people have interfered with their movements. They were given asylum and paid social benefits even if they had been sentenced for terrorist activities in their home countries. In this respect, Britain followed the most liberal policy in Europe.

Many Britons believed that the country would be safe if it showed tolerance towards radicals and suspected terrorists. No one went further in this respect than Ken Livingstone, the London mayor, who welcomed some of the gurus of the "militants" and claimed that they were harmless religious dignitaries maligned by Zionists and neoconservatives. George Galloway, Saddam Hussein's greatest admirer this side of Baghdad, was recently elected to the British parliament from a racially-mixed precinct in East London....

Governments are responsible for the security of their citizens. Yet they can't move too far ahead of public opinion. As long as it is not generally understood that restrictions and controls will be inevitable in the future to safeguard society from far worse disasters, terrorists will have a (relatively) free run. In the case of future major attacks, public pressure for such measures will be overwhelming. If such attacks do not occur, we shall be able to sleep in peace and quiet, lucky to enjoy both security and all our liberties. The prospects of this scenario coming true aren't brilliant.