The biggest threat we face: the West vs. radical Islam.

INTERNATIONAL terrorism, in its Muslim-extremist variety, is a world phenomenon. In my view it will eventually blow itself out, probably by the collapse of the Muslim world into secularism. But in the meantime it constitutes the biggest threat that Western civilization faces.

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There are various ways of dealing with this threat, all of which are being explored. But the vital way is to localize it, or rather to direct it into areas where it can do least harm to the West. Partly by accident, partly by design, this strategy has been pursued by the U.S., Britain, and their few dependable allies.

The 9/11 attacks, followed closely by the terrorist operations in Madrid and London, were an attempt by Muslim extremists to break out of their traditional localized theater of operations and carry their war into the heartlands of their enemies. One of these operations succeeded. The Madrid attack showed how terror can work politically on a national scale. For the bombing panicked a sufficient proportion of the Spanish people to oust a conservative government which was committed to supporting America's operations, and replace it with a quasi-pacifist left-wing one which is willing to appease the terrorists.

The operations against the American and British heartlands failed, however. Not only did both nations place themselves on armed alert at home, in a way that has made it difficult to repeat 9/11-style attacks, but both also determined to carry the counteroffensive into the terrorists' heartlands by toppling regimes which supported them, the Taliban in Afghanistan and Saddam Hussein in Iraq. The Allied occupation of these two Muslim countries, and the accompanying attempt to install democratic institutions there, has been criticized in the Western media, and by the appeasers of the European Union, as unsuccessful, inconclusive, wasteful, clumsy, and costly in life. Some of these criticisms are justified in part. But what is left out is the success in transferring the theater of conflict from the West to the Middle East.

Essentially, the fighting is now taking place in the Arab and Muslim worlds. It is being conducted on our side by American and British professional armed forces, using all their advantages in military technology. Of course there have been casualties, and no doubt there will be more. But they are very few in comparison with wars of comparable political importance, and we are learning how to minimize them.

The important result is that the civilian populations of big Western cities are no longer on the front line. It has shifted to the Middle East. Any destruction is taking place in Muslim territories. The overwhelming majority of lives lost are Muslim. Fanatics all over the Muslim world who wish to become active terrorists--or "fighters," as they call themselves--instead of infiltrating Western cities and forming cells there, have concentrated in Iraq, where they have been killed by Allied firepower in huge numbers. They have been putting themselves where we want them to be: in direct opposition to professional armies.

Inevitably, this re-concentration of Muslim extremism in the Arab Middle East has reignited the Israeli-Arab conflict, or rather opened a new and more intense phase. That in itself demonstrates the global nature of the terrorist threat and the importance of Israel as the front line and lightning rod of the West. George W. Bush is quite right to regard Israel's security as part of the West's security, to regard Israel's efforts to uphold that security by air strikes and cross-border army deployment with sympathy, and to supply any weapons which are needed to make them more effective. The more terrorist manpower, weapons, money, and emotional resources go into Hezbollah and similar organizations, where they have to emerge on the surface and are exposed to Israel's overwhelming firepower, the less will be available for covert, underground operations against the West, in the West, and with massive casualties among Western civilians. Hence Israel's security operations in Gaza, and still more in Lebanon, reinforce the consequences of the Allied occupation of Afghanistan and Iraq--that is, they help to confine the main terrorist threat to the Middle East, and to pit the terrorists themselves against professional soldiers, Israeli, American, or British.

If we look at the campaign overall in terms of cost-effectiveness, we see that it is far preferable to do things this way. It is better, easier, less costly in enemy and Western lives, to fight the battle on grounds of our choosing, in an offensive spirit using our superior weaponry, than to fight a defensive war back home.

The other new development, as a result of the U.S.-British transfer of the struggle to the Middle East and the eclipse of Saddam Hussein, has been the emergence of the fanatical Iranian regime as the main refuge, training ground, arms supplier, and financer of Muslim terrorism. Attempts have been made, and are being made, to overturn the regime from within. But they are unlikely to succeed, and sooner or later--probably sooner--the Iran menace will have to be confronted directly.

Oddly enough, Iran's blatant pursuit of nuclear weapons makes the problem easier to deal with, both morally and physically. So long as Iran's backing of international terrorism is confined to supplying terrorists with conventional weapons--even such formidable ones as the medium-range missiles given to Hezbollah for use against Israeli cities and Civilians--toppling its regime poses formidable political, diplomatic, and military difficulties. Iran is a huge country of many different regions, all savagely inhospitable (I have been all over it), as Alexander the Great discovered over two millennia ago. Occupying it effectively would be almost impossible.

On the other hand, crippling it from the air in such a way as to prevent the regime from doing much harm to the West is possible, provided we are prepared to notch up the firepower a few points. The question is: How far?

If Iran succeeds in producing a few nuclear weapons, there is absolutely no doubt that it will use them immediately, both in delivering them by aircraft, rocket, or other means in Israel, and by giving them to terrorist organizations to use, in whatever way possible, against Western cities. We have to assume, therefore, that catastrophe can be averted only by preventing Iran's acquiring nuclear capability. This means preventive strikes, and the only real question is: With what weapons can we ensure that these strikes are effective and end the Iranian nuclear threat once and for all?

There is no doubt that, faced by the imminent threat of a nuclear attack made possible by the Iranian regime, Israel would use her own nuclear arsenal in a preemptive strike. That is what it is for, as a last resort against the total annihilation of the Israeli people, to which of course the leaders of the Iranian regime are committed. In such circumstances, Israel would have our support, and if necessary the active cooperation of Anglo-American armed forces.

But would it not be more effective, and honest, for America and Britain to join, at the outset, any effort to emasculate Iran and prevent its regime from inaugurating nuclear war? And in this preemptive war, should we not be prepared to use nuclear weapons ourselves, to make the destruction of the Iranian nuclear threat absolutely complete and instantaneous?

These are formidable and frightening questions. But they are at the back of the minds of those who think seriously about the state of the globe, and it is better that they be brought into the open and discussed calmly and realistically.

Mr. Johnson is the author of many books, the most recent of which is Creators: From Chaucer and Durer to Picasso and Disney.