WASHINGTON - The Barack Obama administration's declaration in its Nuclear Posture Review (NPR) that it is reserving the right to use nuclear weapons against Iran represents a new element in a strategy of persuading Tehran that an Israeli attack on Iranian nuclear sites is a serious possibility if Iran does not bow to the demand that it cease uranium enrichment.

Although administration officials have carefully refrained from drawing any direct connection between the new nuclear option and the Israeli threat, the NPR broadens the range of contingencies in which nuclear weapons might play a role so as to include an Iranian military response to an Israeli attack.

A war involving Iran that begins with an Israeli attack is the only plausible scenario that would fit the category of contingencies in the document.

The NPR describes the role of U.S. nuclear weapons in those contingencies as a "deterrent". A strategy of exploiting the Israeli threat to attack Iran would seek to deter an Iranian response to such an attack and thus make it more plausible.

The new nuclear option on Iran has emerged after a series of public statements over the past year by senior officials, including Secretary of State Hillary Clinton and Vice-President Joe Biden, suggesting the administration would tolerate an Israeli option.

Both the Bill Clinton and George W. Bush administrations had said the United States "reserves the right" to respond with nuclear weapons to the use of chemical and biological weapons in an attack on U.S. forces or its "friends" or "allies".

A contingency plan called CONPLAN 8022-02, adopted in November 2003, aimed at destroying an adversary's nuclear weapons or nuclear facilities, included the option of using earth-penetrating nuclear weapons to destroy deeply buried facilities.

But the new NPR refers to "a narrow range of contingencies in which U.S. nuclear weapons may still play a role in deterring a conventional or CBW attack against the U.S. or its allies or partners".

That language appears to suggest that the nuclear option would deter an Iranian conventional retaliation against Israel or U.S. military targets in the region in the event of an Israeli air attack on Iran.

Both Obama and Defense Secretary Robert Gates made statements implicitly linking the new nuclear declaration to the broader problem of trying to force Iran to bow international demands on the nuclear issue.

Interviewed by CBS News Apr. 1, Obama was asked what made him think sanctions would work this time. After referring to Iran's isolation, which he said would eventually "have an effect on their economy", Obama made an obvious allusion to military options. "Now, you know, I have said before that we don't take any options off the table," said Obama, "and we're gonna continue to ratchet up the pressure and examine how they respond."

In the past, references to options being on or off the table had been used to refer to the option of a conventional U.S. air attack. In this case, however, Obama was clearly referring to the announcement of the nuclear option in the NPR that he knew was coming on Apr. 6 as a way to "ratchet up the pressure" on Iran.

Asked in an interview with the New York Times Apr. 5 whether he believed Israel would decide to attack Iran if it "stays on the current course", Obama refused to "speculate on Israeli decision-making".

But he said,"[W]e want to send a very strong message both through sanctions, through the articulation of the Nuclear Posture Review, through the nuclear summit that I'm going to be hosting, and through the NPT review conference that's going to be coming up, that the international community is serious about Iran facing consequences if it doesn't change its behavior."

Gates was even more pointed in highlighting what he called the "message for Iran" in his Apr. 6 news briefing on the NPR, saying that "all options are on the table in terms of how we deal with you".

It was not the intention of the original drafters of the NPR within the State Department to issue a new threat to Iran, according to a source who was briefed on the NPR earlier this month. But the official involved in the drafting acknowledged that Gates and Obama had seized on the language to suggest that the United States now had a stronger hand in dealing with Iran, according to the source.

The White House Coordinator for WMD, Counter Terrorism and Arms Control is Gary Samore, who had had publicly discussed the need to exploit Iranian fear of an Israeli attack to gain diplomatic leverage over Tehran before joining the Obama administration.

At a forum at Harvard's Kennedy Institute in September 2008, Samore had said that the next administration would not want to "act in a way that precludes" an Israeli attack on Iran, "because we're using the threat as a political instrument".

Samore was asked during the question and answer session after a speech at the Carnegie Endowment for International Peace in Washington Wednesday whether he expected Iran to believe that the United States would use nuclear weapons against Iran if it retaliated with conventional weapons against an Israeli attack on Iran. Samore ignored the question in answering.

As part of an apparent effort to make Iran uncertain about an Israeli attack, a series of public statements by U.S. senior officials over the past year have suggested that the would do nothing to prevent such an Israeli attack. However, the Obama administration has conveyed to the Israeli government privately that it strongly opposes any Israeli attack on Iran, according to reports in the Israeli press.

A former senior U.S. intelligence officer on Iran believes the nuclear option is likely to cause Iran to go farther in the direction of nuclear weapons rather than to give in. In an e-mail to IPS, Paul Pillar, who was the national intelligence officer for Near East and South Asia from 2000 to 2005, said Iranian officials probably see the new nuclear option as "another manifestation of U.S. hostility toward Iran".

The perception of a U.S. threat to Iran "provides one of the principal incentives for Iranians to develop their own nuclear weapons", said Pillar.

Pillar said Iranians "may also see the doctrine as providing cover for an Israeli strike by serving as a deterrent against Iranian retaliation for such a strike".

Other political-military analysts cast doubt on the credibility of the announced nuclear option against Iran.

Morton Halperin, who was director of Policy Planning in the State Department in the Clinton administration, told IPS, "I don't think it's credible at all. I don't think the administration thinks it's credible. But I think as a political matter, to have taken it off the table would have been politically untenable."

Jim Walsh of the MIT Security Studies Program, who has had many contacts with Iranian leaders and national security officials in recent years, told IPS the United States "is not going to use nuclear weapons against Iran" and that it is "foolish" to suggest that "all options are on the table".

Gareth Porter is an investigative historian and journalist specializing in U.S. national security policy. The paperback edition of his latest book, "Perils of Dominance: Imbalance of Power and the Road to War in Vietnam", was published in 2006.