In France, authorities have been caught by surprise again and again by terrorist attacks with many lives lost; in Belgium, terrorists have been coming and going for years, buying military weapons with remarkable ease. But it is Italy that has long been the most vulnerable. It has been a front-line state in all the recent military expeditions in the Muslim lands of Afghanistan, Iraq and Libya; it has added to the provocation by loudly supporting U.S. policies and by actively collaborating with Israel, lately on multibillion-dollar military projects that have received much publicity.

Moreover, Italy has a Muslim population that exceeds 2 million and porous borders -- and lately no borders at all because of the mass entry of Muslims from Syria, Turkey, Iran, Afghanistan and Pakistan, a country where more than 60% of the population supports the Islamic State group.

Yet nobody has been killed by Muslim terrorists in Italy.

This is not because no one has tried. Since Sept. 11, 2001, when they went on full alert, never to stand down again, Italian Foreign Intelligence, internal security, the carabinieri gendarmerie and police forces have detected and interrupted hundreds of terrorist plots large and small, at every stage from mere verbal scheming to fully ready actions, of the kind successfully completed in Madrid, London, Paris, Toulouse, Copenhagen, Brussels and elsewhere.

Only in Italy have plotters failed to become terrorists, even though the Vatican is actually the most iconic target in Europe, and now the top target of the Islamic State, judging by its videos.

Italy is fortunate in its natural and man-made beauties, but its success in counterterrorism so far -- fingers crossed -- owes nothing whatever to good luck.

It is all a question of method, itself derived from one fundamental insight: Terrorist actions cannot be anticipated and prevented -- all such efforts are simply futile because there are just too many possible targets and an infinity of possible dates. Nor can one hope to detect even imminent attacks because terrorists need not reveal themselves until it is too late.

Actually there is only one thing that can be done: to follow all who are suspected to be truly dangerous 24/7 so that they can be arrested or killed no matter how suddenly they emerge to attack.

This, however, is quite impossible unless the total population of suspects -- in the hundreds at any one time in the case of Italy, even into the thousands sometimes -- is drastically reduced to enable the allocation of enough security personnel. Figure at least four for each truly dangerous suspect as well as teams of two dozen or more for each possible plot.

It is at this point that the Italians differ drastically from the French or Belgians or British, who keep monitoring suspects, filling ample files with lots of reports, photographs, videos, and intercept print-outs, as if they were biographers of the most encyclopedic kind.

The Italians by contrast take action the moment the very first indication comes in. This is sometimes a phone or e-mail intercept, and sometimes a tip-off from an agent (Italy invests modestly but steadily in the foreigners it recruits). But more often it is a friend who reports someone for speaking of jihad, for swearing that he will kill for Hamas, join the Islamic State or do something else.

What follows is an expert interrogation. Many are soon sent home, classified as mere boasters if they turn meek. Those who are proud to be militant are held, and their records are minutely examined to find any criminal infraction for which they can be arrested, tried and imprisoned (unlike the French who treat militant petty thieves as petty thieves, not as militants). If any faults are found in the immigration paperwork, they are deported -- even if they have become Italian citizens -- a measure applied to several vehement imams (181 more imams are currently in prison).

With these methods, the number to be followed 24/7 falls to manageable levels -- the key to the success of the entire system.

In the U.S., the FBI's method is to "help" suspects by sending agents to offer them weapons, explosives, etc. If they refuse and call the police, they are taken off the suspect list. But if they start preparing an attack, the FBI arrests them, and 20-year sentences for attempted terrorism follow. The French complain that they have too many suspects, that they cannot keep track of thousands of potential terrorists. The Belgians say the same of their hundreds of suspects. But it is their fault. They have refused to use either the Italian or U.S. method to reduce the number of suspects one by one, one way or the other. It remains to be seen if they will now change their ways.

Edward N. Luttwak is a senior associate for the Center for Strategic and International Studies. He has served in the U.S. as a consultant to the Office of the Secretary of Defense, the National Security Council, the U.S. Department of State, the U.S. Army, Navy and Air Force. He also has advised a number of allied governments as well as international corporations and financial institutions.