The paradox of U.S. policy toward Iran is that no one, including the Bush administration, wants to use military force. Yet neither Tehran nor our European allies have sufficient incentive to stop Iran's nuclear program peacefully without a reasonable belief that the United States will use force if diplomacy fails.

Those who oppose military action rightly point to the risks involved. Iran would retaliate and probably with some success. Any military action would prompt angry denunciations even from close allies. International condemnation might be tolerable if the action accomplished its purposes. But, even after a successful attack, the United States would not know how much damage it had done to Iran's nuclear program and would have a difficult time monitoring the Iranians' inevitable efforts to resume it. The only answer to both problems would be to remove the regime entirely and install international monitors. But that would require at least temporary occupation of the country and the use of hundreds of thousands of ground troops that the United States does not currently have.

For these and other reasons, military action really should be the last resort. By far the best option remains the marshalling of international political and economic pressure against Iran so as to isolate and impoverish the ruling elite and strengthen the hand of those who already may be questioning the wisdom of the current path. There are signs these pressures have caused concern in Tehran, though not enough to change its course. More pressure and isolation could possibly convince Iranian leaders to delay or even suspend their program.

Unfortunately, the prospect of greatly increased international pressures, absent a credible military threat from the United States, is not bright. There is little will among the EU-3--Great Britain, France, and Germany--to move toward strict and punitive economic sanctions against Iran and a growing unwillingness, especially in Berlin, to act without Russian approval. If Russia refuses to isolate Iran, the leading European powers are unlikely to act on their own.

European reticence poses problems in the long as well as short run. Some American analysts believe the United States, rather than taking military action, should lay the groundwork for international containment of Iran once it acquires nuclear weapons. But they overestimate the international community's enthusiasm for such a policy. Russia will not participate in the containment of a nuclear Iran, and the same Europeans who are reluctant to confront Iran now will be even more reluctant to confront it when it has nuclear weapons. This is partly because the majority of Europeans do not believe they have anything to fear from Iranian nuclear arms--unless they provoke Tehran. French President Jacques Chirac's admission that he could tolerate Iran's possession of nuclear weapons accurately reflects the European consensus.

In fact, to the degree that Europeans are engaged on the Iran issue at all, it is not because they fear Iran so much as because they fear the prospects of a U.S. attack on Iran. The primary aim of the EU-3 talks over recent years has been not to stop Iran's program but to forestall a U.S. action Europeans consider fraught with peril. Paradoxically, or perversely, the Bush administration's continued warnings of possible military action have long provided the strongest incentive for continuing European diplomatic efforts.

For all these reasons, the prospect of a U.S. attack on Iran to prevent its acquisition of a nuclear weapon must be more, not less, credible than it is today. This will require an increase in U.S. military capacities, especially a rapid and significant increase in the overall size of U.S. ground forces. But it will also require substantial political preparation. The present administration has lost credibility with the American public and the world. Were it to claim that Iran is on the verge of building a nuclear weapon, even if intelligence supported such a judgment, most observers would be skeptical. For this and many other reasons, one hopes that Iran will not reach that stage during Bush's presidency. Still, the credibility problem will persist beyond Bush. This administration should begin now to foster greater confidence that the United States will act only on the best available intelligence.

One way to do this is to create yet another bipartisan panel of respected "wise men" to provide regular assessments to the president on the progress of Iran's nuclear program. Bush or any future president would retain the right to take action, or not take action, regardless of the panel's assessment. But ignoring the panel would carry high political risks. The panel itself could be trusted to make a sober judgment of the available facts, inasmuch as it would not want to be held responsible either for the emergence of a nuclear Iran or for a war not justified by the evidence. The virtues of creating such a panel--which might include such senior statesmen as Harold Brown, Brent Scowcroft, Henry Kissinger, George Shultz, and William Perry--are twofold: It would provide reassurance that the United States would not act rashly, and it would also signal American seriousness and determination to act if Iran is found to be on the verge of building a nuclear weapon.

A more credible military option opens the door for more flexible diplomacy. To the extent the United States can establish its ability and willingness to take military action when it becomes necessary--and to the extent that it can build support for such action at home--it will then be free to pursue all possible diplomatic paths to avoid such action, including direct and unconditional negotiations with Tehran. The main reason the United States avoids such talks now, without a prior suspension of Iranian efforts to enrich uranium, is that it fears Iran will simply drag out talks while it proceeds with its nuclear program. But Iranian stall tactics become far less effective if the United States has already signaled that it is willing to take military action to prevent Tehran from acquiring nuclear weapons.

There is bipartisan consensus in the United States that Iran should not be allowed to build a nuclear bomb. A nuclear-armed Iran under the present regime will inevitably grow bolder in pursuing traditional Iranian hegemonic ambitions in the Middle East as well as the ideological mission of spreading its version of Islamic fundamentalism. It will be freer to take aggressive action against neighbors, especially by supporting terrorist groups, knowing that the United States will be wary of confronting a nuclear power. The resulting combination of increased Iranian ambition and the loss of confidence in American resolve will force Iran's neighbors either to accommodate Tehran's rising power or to build nuclear weapons themselves to safeguard their own security and independence. Given Sunni fears of a Shia bomb, the latter option seems likeliest, which would mean a new, many-sided nuclear arms race in the world's most unstable region.

It is not enough to ask whether there are enormous risks and dangers in taking military action before this sequence of events unfolds. There are. But history teaches that the choice is often not between war and no war. Even if the United States chooses not to take military action in the coming years, it might well have to fight later under even more disadvantageous circumstances. The best way of avoiding war is to create a credible threat of military action and then proceed with steps that could produce a peaceful resolution of the crisis--vigorous international diplomacy and stronger sanctions. But the military option needs to be viable--not least because, as the recent hostage crisis involving British sailors makes clear, Iran itself may start a war.

Robert Kagan is a contributing editor at The New Republic and the author of Dangerous Nation: America's Place in the World from its Earliest Days to the Dawn of the Twentieth Century.