The French president threatens nuclear retaliation against state sponsors of terror. His Russian counterpart makes thunderous vows to strike first at any threats to Russia anywhere on the planet. The Ethiopians, with Western assent, invade Somalia and avert a possible Islamic regime there. The Japanese debate whether to violently eradicate North Korea’s growing nuclear threat. (“Just to be on the receiving end of the attack,” said the Japanese foreign minister, “is not what our constitution had in mind.”) The Australians call for amending the UN charter to allow strikes on terrorists. The British prime minister chides those who wish “to err on the side of caution” and “weigh the risks to an infinite balance.” And the Americans publish a National Security Strategy in which they flatly argue for the need “to stop rogue states and their terrorist clients before they are able to threaten or use weapons of mass destruction against the United States and our allies and friends.”

For many years now, and especially since 2001, several world leaders and prominent defense intellectuals have issued these kinds of unusual, even startling pronouncements and conclusions.

What’s going on here?

Welcome to the new age of preventive war, in which states and their leaders will no longer be content to wait for signs of imminent attack in order to strike first, and will choose instead to eradicate even potential threats long before they can fully ripen into major terrorist attacks or mature into working weapons of mass destruction. Gone are the days of relying on the reassurance of deterrence; gone, too, is passive trust in the good intentions or even in just the sober rationality of potential opponents. For better or worse, the old rules of international politics, the traditional norms and laws that at least tried to govern the use of international violence, are fading away--and fast.

Preventive war is distinct from “preemptive” war, which is a kind of spoiling attack against a recognizable danger that is actually acceptable under current international traditions and laws. International law texts, for example, usually present Israeli action against Arab forces in the 1967 Six Day War as the classic case of justifiable preemption. Prevention, however, is quite different: it is snuffing out an enemy’s capability for harm far in advance of any possible conflict, and based not on self-defense, but on raw calculation of self-interest.

Prevention is not a new concept. Sparta’s decision in the 5th century BC to wage war on Athens was the result, as Thucydides tells us, not of any specific threat to Sparta but out of a more general fear of the “rising power of Athens.” In modern times, the Japanese attack on Pearl Harbor in 1941 was not an act of immediate self-defense, but rather was aimed at neutralizing U.S. power in the Pacific while the odds were still in Japan’s favor, a classic case of a preventive strike. And many argue—with significant justification—that the U.S.-British attack on Iraq in 2003 was a preventive war, designed to destroy notional, rather than actual Iraqi capabilities that could threaten Western security, as President George Bush said at the time, a year, five years, or even ten years later.

Because it responds only to notional, rather than actual, threats—in effect punishing the enemy for what it might do—preventive war has traditionally been regarded as both unwise and immoral. (Bismarck famously said that launching a preventive war was akin to “committing suicide for fear of death.”) But this prohibition against preventive military action is collapsing in a world where unstable and aggressive actors, including terrorists and bizarre entities like North Korea, care little about traditional rules of war. Without warning and without remorse, such actors seem determined to attack the international status quo, even if it means killing thousands of civilians or possibly even starting a nuclear war. It is unfortunate, even tragic, but also understandable, if people around the world are losing their tolerance for any kind of risk. As understandable as it may be, however, it is still a dangerous impulse. If the great powers simply choose to launch preventive military actions at will, large or small, then a new age of preventive war threatens to unleash chaos on the international community.

Two reactions to this new advent of prevention are denial and blame. Neither are useful. Those who still believe that the United Nations, as it is currently constituted, is still somehow capable of controlling the use of force in the 21st century are deluding themselves. The UN has already been sidelined, as operations ranging from Kosovo to the Iraq war have painfully illustrated. And there is no point in trying to assign blame for the collapse of old norms. There is no single nation or leader to blame for the steady erosion of previous rules of war. Indeed, the idea of “blame” is irrelevant: traditions and rules have fallen victim not to politicians but to technological and social changes beyond the control of even the most powerful nations.

The old rules of war are not coming back. Prevention is here to stay. The question now is what to do about that.

There is probably no way to eliminate the prevention temptation, and it’s not clear that we should even want to do so. The world has suffered a parade of atrocities since the end of the Cold War, including murder and destruction on a previously unthinkable scale, from the butchering of schoolchildren to campaigns of rape, even genocide that were supposed to have been buried in the ashes of World War II. The blood and treasure spent to destroy the Axis and then win the Cold War was not intended to finally make the world safe for genocidal maniacs and irresponsible nuclear aspirants, and there are reasonable arguments for acting well in advance of potential horrors.

On the other hand, without some sort of international governance over the use of preventive force, the world will descend into the worst anarchy since the collapse of the League of Nations. No one wants to live in a world where the great powers run about like rogue elephants, stomping out every mouse that frightens them. While some have suggested creating new international institutions to govern the use force composed of like-minded democracies, these would probably be viewed as little more than cover for the United States and its closest allies to do as they please. Instead, the more likely but painful answer probably lies with the United Nations, and with reform in particular of its deeply dysfunctional Security Council. The Council is a body whose organization and membership makes little sense in the 21st century. Changing it, especially if this means weakening the sacred veto, or rejecting the bid for membership of violent dictatorships, will be arduous and controversial. But without any control over the use of force against even legitimate threats, order in the future will be maintained not by “coalitions of the willing,” but by (in the words of a British general) “coalitions of the exasperated,” Wild-West posses of the most militarily capable powers who will see to it that their interests and the lives of their peoples are not threatened. That might, in some ways, produce a more physically secure world in the short term. But it is not a world that would reflect American or democratic values, and it would not, in the end, be a world worth fighting for.