Any residual doubt that Washington has decided to take no action to prevent Iran from acquiring nuclear weapons dissipated Wednesday with the news that Undersecretary of State William Burns will be participating in EU foreign policy chief Javier Solana's negotiations with Iran's nuclear negotiator Saeed Jalili in Geneva on Saturday.

That those negotiations will fail to end or even slow Iran's progress toward nuclear weapons capabilities is a certainty. Ahead of the talks, Iran's Supreme Leader Ayatollah Ali Khamenei reiterated for the umpteenth time that Teheran will make no compromises on its uranium enrichment activities. And so far, Iran - as opposed to Washington - has been true to its word.

Given Iran's forthrightness, there is only one reasonable explanation for the administration's decision to send Burns to meet with Jalili: The US wants it to be absolutely clear to Teheran and everyone else that it has no intention whatsoever of attacking Iran's nuclear installations.

It makes sense that Washington considers it necessary to make this point clearly. In light of the threat that a nuclear-armed Iran would constitute to US national security interests, it would have been more reasonable to assume that America would attack the Islamic Republic's nuclear facilities preemptively than to assume it would allow Iran to go forward with its goal to acquire nuclear weapons.

A nuclear-armed Iran would place the US military's hard-won victories against Iranian surrogates in Iraq and its tentative success in separating Iraq's Shi'ite leaders from Teheran in jeopardy. So, too, given Iran's increasingly active support for the Taliban, an Iranian acquisition of nuclear capabilities would cast doubt on America's ability to defeat the resurgent Taliban.

The US's economic well-being would also be endangered by a nuclear-armed Iran. Teheran has repeatedly threatened to attack Saudi oil platforms and endanger the oil shipping lanes in the Straits of Hormuz. And a nuclear arsenal would give Iran unprecedented power to dictate price-setting policies for the OPEC oil cartel.

Beyond all that, a nuclear-armed Iran would directly threaten US territory in two ways. First, there is no reason not to think that Teheran would use Hizbullah cells in the US to detonate nuclear devices in US cities. Iran has already shown a willingness to use Hizbullah to carry out terror attacks in the West - most spectacularly in the 1994 bombing of the AMIA Jewish community center in Buenos Aires.

Second, it is widely feared that Iran is developing the capacity to launch an electromagnetic pulse (or EMP) attack against the US mainland. An EMP attack is conducted by launching a nuclear bomb into the atmosphere above a country. It needn't actually hit the country. Simply by detonating a nuclear device at sufficiently high altitude, an EMP attack can destroy the electrical grids, communications systems and military-industrial foundations of a society. Such an attack would set the US back a hundred years.

Fears of an EMP attack against the US were sparked last week by Iran's test of an advanced version of its Shihab-3 ballistic missile. The day of the missile test, William Graham, who heads a congressionally mandated commission on the EMP threat to the US, gave testimony on the issue to the House's Armed Services Committee. Graham explained that Iran has already conducted missile test from ships in the Caspian Sea. If it acquires nuclear weapons, it will apparently have the capacity to launch a nuclear warhead capable of carrying out an EMP attack against the US from a freighter in international waters off the US coast.

While any of these threats would be sufficient to justify a preemptive attack against Iran's nuclear installations, the US still has a reasonable excuse for not conducting such an attack: Iran has made clear that if it acquires nuclear weapons, the US will not be Teheran's first target. Israel enjoys that distinction.

And since the US is Iran's second target, the Bush administration has made clear that if Iran attacks Israel, the US will launch an attack against Iran. That is, the US will fight to ensure that Iran won't be able to attack it if America moves to the head of Iran's target list. But as long as it's only No. 2, it will take no action.

The US cannot be accused of being unfair to Israel by deciding not to prevent Iran from acquiring nuclear weapons. After all, defending Israel is Israel's responsibility, not America's. And on this point, news reports in recent weeks have made it clear that while the US will not attack Iran, it has given Israel a "green light" for a preemptive strike on the Islamic Republic's nuclear installations. And this is no small thing.

THE BUSH administration's willingness to stand back and allow Israel to attack Iran's nuclear installations to prevent a nuclear holocaust of the Jewish state compares well with how the administration of the president's father treated Israel in the 1991 Gulf War. At that time, Israel was under threat of Scud missile borne chemical weapons attack. Although Saddam Hussein ended up not attacking Israel with chemical weapons, the threat that he would was credible. He attacked Israel with Scud missiles almost every night for the duration of the war.

Despite this obvious casus belli, the first Bush administration not only refused to politically support Israel's right to defend itself against Iraqi aggression, it took active steps to prevent Israel from attacking Iraq's Scud missile installations. Then-president George H.W. Bush refused to provide Israel with the electronic codes that would allow Israeli and US jets to identify one another as friendly aircraft. In so doing, he left open the prospect that the US would shoot down IAF jets over Iraqi airspace if Israel dared to defend itself.

So, mindful of the precedent set by his father, President George W. Bush's decision to leave the door wide open for an Israeli preemptive strike on Iran is a positive development. But an open door is only significant if someone is willing to walk through it. And it is far from clear that the Olmert-Livni-Barak-Yishai government has any intention of doing so.