With the Iranian nuclear crisis about to land in the Security Council, the events that led up to the war in Iraq point clearly to what needs to be done.

In the decade preceding the Iraq war, Saddam Hussein was able to defy the major powers when they were divided. Exploiting his advantage as a single actor and Washington's indecision over whether it most wanted nonproliferation or regime change, Saddam consistently outmaneuvered his diplomatic opponents.

But he also knew that he could not buck the five permanent members of the United Nations Security Council when they were serious and united (as, for example, during the toughened United Nations inspections in the months just before the war), and he did not even try.

The situation with Iran today is the same. Washington cannot decide whether the top priority of its Iran policy should be regime change or nonproliferation; as a result, other major powers do not trust and will not fully support its anti-nuclear efforts. Tehran has easily exploited this lack of unity among the major powers to frustrate every attempt to rein in its nuclear program. It has recruited international support through energetic diplomacy to the group of developing countries known as the G-77. Just as Saddam did, the leaders in Tehran have made diplomatic mincemeat of the big powers.

Iran has been so successful in this that some experts in the West have turned defeatist. Their view is mistaken. The idea that "it's too late" to stop Iran's progress toward building nuclear weapons is technologically wrong. "There's nothing we can do about it" ignores a range of options between economic sanctions, going to war and more of doing nothing. And the argument that "we can learn to live with it" misses the nature of the challenge.

A nuclear Iran is dangerous enough, but this crisis is only proximately about Iran. More important, it is about the likely consequence of an Iranian bomb, namely, that Saudi Arabia, Turkey and Egypt would produce their own bombs, and, thanks to the concomitant international failure to deal with North Korea, the nonproliferation regime would collapse. What is at stake is not a choice between nine and 10 nuclear weapons states, but a choice between nine and 30 or more.

The major powers may yet be able to unite to stop Iran at this late hour, but not without a decisive change in American policy. Washington's choice is simple: Does it want to stop Iran's acquisition of nuclear weapons badly enough to deal with Iran's present government?

Like the Clinton administration with Iraq, the Bush administration has been unable to make up its mind, locking its policy into a cycle of self-defeating half-steps. The combination of "axis of evil" threats, partial support of European negotiations and publicly announced financial backing for the opposition in Iran ensures only that we will fail either to negotiate an end to the nuclear program or to overthrow the present regime.

The administration must, finally, hold its nose and recognize that the nuclear challenge is the indisputable priority. It must get off the sidelines and into negotiations with Tehran. It must solidify agreement among its fellow permanent council members by working closely with Russia, not least by concluding a long-overdue pact on civil nuclear cooperation. Russian participation would make it possible to provide Iran with a credible international guarantee of uranium enrichment and reprocessing services.

With China and the others, the United States needs to make clear that the Security Council can resort to other steps besides economic sanctions to significantly raise the cost to Tehran of its continued defiance, beginning with making International Atomic Energy Agency inspections mandatory rather than voluntary.

Secretary of State Condoleezza Rice (and her fellow foreign ministers from the council's permanent members) should be flying to Brazil, Indonesia, South Africa and other leading countries of the G-77 to explain why Iran is wrong to claim that the Nuclear Nonproliferation Treaty gives it the "right" to enrich uranium, and why Iran's abuse of the treaty devalues each of their commitments to give up nuclear weapons.

Given the American record with Iraq and Iran, others will be skeptical that Washington has made a clear choice for nonproliferation and away from regime change. The message will have to be steady and unequivocal. If President Bush and Secretary Rice continue to say one thing and Vice President Dick Cheney and our ambassador to the United Nations, John Bolton, say another, the effort will quickly fail.

Members of Congress have a direct responsibility as well. Only they — especially the Democrats — can make such a policy change possible. They will have to forgo the indulgence of slamming the administration from the right and currying favor with pro-Israel voters by vying to see who can be the most anti-Iranian.

All of this, and more, is what serious anti-nuclear diplomacy would look like. It has not yet been tried. Anyone who promotes the use of military force from the present position of American indecision and before the obvious political steps have been taken is repeating the error that led us into Iraq.

The international community's record on Iran's nuclear program (as on North Korea's) has been feckless. Only the United States can change that. If we fail to pursue this effort with unwavering, clear-minded diplomacy, a nuclear-armed world will be the Bush administration's chief legacy, no matter how the war in Iraq and the war on terrorism turn out.

Jessica T. Mathews is president of the Carnegie Endowment for International Peace. She wrote this commentary for The New York Times.