LONDON — “I will not wait on events while dangers gather.” Thus speaks U.S. President George W. Bush — and in doing so appears to state, in plain and simple language, a revolutionary new doctrine that upends five decades of thinking about global security.

For what the president is saying — and it sounds quite reasonable after the terrible events of Sept. 11 — is that the United States will strike first. It will never again wait passively to be attacked and then hit back. That would be too late, as it was too late for the thousands who died that fateful day in New York.

But strike first with what and at what? If the president is saying that America will ruthlessly hunt down terrorists and search out their training camps, wherever in the world they may be, with agile new super-mobile forces, that is not a new intention. If he is saying that America’s vast conventional forces will be launched against regimes that harbor and succor terrorism of the al-Qaeda variety, that, too, sounds familiar and predictable, even if it raises severe logistic problems (how, for example, do you march on Baghdad without unacceptable casualties all round?).

But if Bush is talking about pre-emptive strikes with nuclear weapons, then that raises far bigger issues. Would these be strikes against cities and countries or against terrorist bases, missile installations or weapons-development facilities? And would the occasion for them merely be that the target country was fostering terrorists with deadly plans against the West, or actively developing nuclear and/or biological weapons of mass destruction (which, in the nuclear case, would include India, Pakistan and Israel, for a start), or actually demonstrating an intent to use such weapons against the U.S.?

Nor do the questions stop there. What kind of nuclear weapons are being contemplated: monster hydrogen-bomb warheads or shorter range “tactical” missiles — if, indeed, any realistic distinction can be made between these categories?

And most important of all from the European point of view, does the new thinking extend to the rest of NATO, which until now believed itself to be firmly under the umbrella of nuclear response in the event of attack? Would a third country’s threat of attack on part of Europe lead to such a pre-emptive first strike?

The old Cold War theory was straightforward enough — if a Western nation was attacked with nuclear weapons, of any size or shape, the existing nuclear powers would respond in kind, thus ensuring total nuclear destruction for both parties. This was the classic deterrence doctrine of Mutually Assured Destruction, so terrible in prospect that no nation or government would ever try it.

The proponents in Washington of the new strategy of pre-emption (if that is what it is) would argue that when the enemy is neither a government nor a nation, and when there is no territory or homeland to defend, then MAD no longer works. Hence the more ferocious threat of first strike.

But could such a threat, as applied to nuclear weapons, be credible? Could any democratic government ever take such a first step to use nuclear weapons, however imminent the threat, or however reliable the intelligence that an attack was coming?

A further complexity is how this approach fits in with the development of antiballistic missile defenses, which were, of course, forbidden, or severely limited in Cold War days, by the ABM Treaty, since they weakened the logic of the balance of mutual destruction.

Bush now seems to have persuaded Russian President Vladimir Putin that this treaty can safely be consigned to the Cold War dustbin and that responsible nations will be better off with the development of antimissile systems, as a kind of reinforcement of the old MAD system — providing, of course, that the immensely challenging technology for picking off missiles at launch or in flight can be made to work.

But by their nature, antiballistic missile defenses are still second-strike systems. The button is only pressed when the other side moves first. They are not pre-emptive in the way that the president is now suggesting.

The position of the other nuclear powers is also complicated by the new American thinking. Basically Britain and France are “simple” nuclear deterrent nations in the sense that, if attacked by nuclear weapons, they have the capability to respond in kind.

But a pre-emptive first strike by the Americans, or even the threat of it, could easily lead to counter-action not against the American landmass but against British and French soil, thus dragging the two countries into a fatally premature nuclear exchange with third countries.

As for China, there can be no knowing how Beijing will respond to the American example. The most likely effect will probably be to increase Chinese determination to keep all nuclear options open, and the rest of the world nervously guessing.

All in all, there really is now an urgent need for the Americans to clarify the meaning of their words and their intentions. If “pre-emptive strike” means going after terrorist networks and cells before they can do their worst, that is a strategy that, with robust American leadership, the global community is already pursuing and should continue to do so. If it means working still harder to prevent nuclear proliferation, that, too, makes sense.

But if it means “nuking” rogue states or other entities before their fingers even reach for the button, that is a strategy that could lead the world swiftly to ultimate disaster.

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