Who could have imagined that alliance management would be a hot election issue? But it is. John Kerry's repeated pledge to restore relations with U.S. allies has struck a chord. The trouble is, if he is elected president, Kerry is going to find that promise hard to keep -- at least with America's allies in Europe. Most of them would be delighted to see Kerry win, but that doesn't mean they will be more cooperative on policy issues. Terrorism is understandably on everyone's mind, but there is yet another growing danger over the horizon. Early into a Kerry administration, we could see a familiar sight -- a transatlantic crisis -- except this time it wouldn't be over Iraq but Iran.

The threat to the United States from Iraqi weapons of mass destruction, if they ever existed, is in the past. Iran, on the other hand, is the problem of the future. Over the past two years, thanks to tips from Iranian opposition groups and investigations by the International Atomic Energy Agency, it has become clear that Iran is seeking to develop nuclear weapons. In the words of the agency, Iran has "a practically complete front-end of a nuclear fuel cycle," which leads most experts to believe it is two to three years away from having a nuclear bomb.

European countries were as worried by this development as Washington, and because the United States has no relations with Iran, Europe stepped in last fall and negotiated a deal with Tehran. It was an excellent agreement, under which Iran pledged to stop developing fissile material (the core ingredient of a nuclear bomb) and to keep its nuclear program transparent. The only problem is, Iran has recently announced that it isn't going to abide by the deal. As the IAEA's investigation became more serious, Tehran became more secretive. One month ago the agency condemned Iran for its failure to cooperate. Tehran responded by announcing that it would resume work in prohibited areas.

That's where things stand, with the clock ticking fast. If Iran were to go nuclear, it would have dramatic effects. It would place nuclear materials in the hands of a radical regime that has ties to unsavory groups. It would signal to other countries that it's possible to break the nuclear taboo. And it would revolutionize the Middle East. Saudi Arabia and Egypt would feel threatened by Iran's bomb and would start their own search for nuclear technology. (Saudi Arabia probably could not make a bomb but it could certainly buy necessary technology from a country such as Pakistan. In fact, we don't really know all of the buyers who patronized Pakistani scientist Abdul Qadeer Khan's nuclear supermarket. It's quite possible Saudi Arabia already has a few elements of such a program.) And then there is Israel, which has long perceived Iran as its greatest threat. It is unlikely to sit passively while Iran develops a nuclear bomb. The powerful Iranian politician Ali Akbar Hashemi Rafsanjani has publicly speculated about a nuclear exchange with Israel. If Iran's program went forward, at some point Israel would almost certainly try to destroy it with airstrikes, as it did on Iraq's reactor in Osirak. Such an action would, of course, create a massive political crisis in the region.

In the face of these stark dangers, Europe seems remarkably passive. Having burst into action last fall, it does not seem to know what to do now that Iran has rebuffed its efforts. It is urging negotiations again, which is fine. But what will it tell Iran in these negotiations? What is the threat that it is willing to wield?

Last month the Brookings Institution conducted a scenario with mostly former American and European officials. In it, Iran actually acquires fissile material. Even facing the imminent production of a nuclear bomb, Europeans were unwilling to take any robust measures, such as the use of force or tough sanctions. James Steinberg, a senior Clinton administration official who organized this workshop, said that he was "deeply frustrated by European attitudes." Madeleine Albright, who regularly convenes a discussion group of former foreign ministers, said that on this topic, "Europeans say they understand the threat but then act as if the real problem is not Iran but the United States."

U.S. policy toward Iran is hardly blameless. Washington refuses even to consider the possibility of direct talks with Iran, let alone actual relations. Europeans could present Washington with a plan. They would go along with a bigger stick if Washington would throw in a bigger carrot: direct engagement with Tehran. This is something Tehran has long sought, and it could be offered in return for renouncing its nuclear ambitions.

But for any of this to happen, Europe must be willing to play an active, assertive role. It must stop viewing itself merely as a critic of U.S. policy and instead see itself as a partner, jointly acting to reduce the dangers of nuclear proliferation. And it should do this not as a favor to John Kerry but as a responsibility to its own citizens and those of the world.