The adviser added, however, that the idea of using tactical nuclear weapons in such situations has gained support from the Defense Science Board, an advisory panel whose members are selected by Secretary of Defense Donald Rumsfeld. "They're telling the Pentagon that we can build the B61 with more blast and less radiation," he said.

The chairman of the Defense Science Board is William Schneider, Jr., an Under-Secretary of State in the Reagan Administration. In January, 2001, as President Bush prepared to take office, Schneider served on an ad-hoc panel on nuclear forces sponsored by the National Institute for Public Policy, a conservative think tank. The panel's report recommended treating tactical nuclear weapons as an essential part of the U.S. arsenal and noted their suitability "for those occasions when the certain and prompt destruction of high priority targets is essential and beyond the promise of conventional weapons." Several signers of the report are now prominent members of the Bush Administration, including Stephen Hadley, the national-security adviser; Stephen Cambone, the Under-Secretary of Defense for Intelligence; and Robert Joseph, the Under-Secretary of State for Arms Control and International Security.

The Pentagon adviser questioned the value of air strikes. "The Iranians have distributed their nuclear activity very well, and we have no clue where some of the key stuff is. It could even be out of the country," he said. He warned, as did many others, that bombing Iran could provoke "a chain reaction" of attacks on American facilities and citizens throughout the world: "What will 1.2 billion Muslims think the day we attack Iran?"

With or without the nuclear option, the list of targets may inevitably expand. One recently retired high-level Bush Administration official, who is also an expert on war planning, told me that he would have vigorously argued against an air attack on Iran, because "Iran is a much tougher target" than Iraq. But, he added, "If you're going to do any bombing to stop the nukes, you might as well improve your lie across the board. Maybe hit some training camps, and clear up a lot of other problems."

The Pentagon adviser said that, in the event of an attack, the Air Force intended to strike many hundreds of targets in Iran but that "ninety-nine per cent of them have nothing to do with proliferation. There are people who believe it's the way to operate"—that the Administration can achieve its policy goals in Iran with a bombing campaign, an idea that has been supported by neoconservatives.

If the order were to be given for an attack, the American combat troops now operating in Iran would be in position to mark the critical targets with laser beams, to insure bombing accuracy and to minimize civilian casualties. As of early winter, I was told by the government consultant with close ties to civilians in the Pentagon, the units were also working with minority groups in Iran, including the Azeris, in the north, the Baluchis, in the southeast, and the Kurds, in the northeast. The troops "are studying the terrain, and giving away walking-around money to ethnic tribes, and recruiting scouts from local tribes and shepherds," the consultant said. One goal is to get "eyes on the ground"—quoting a line from "Othello," he said, "Give me the ocular proof." The broader aim, the consultant said, is to "encourage ethnic tensions" and undermine the regime.

The new mission for the combat troops is a product of Defense Secretary Rumsfeld's long-standing interest in expanding the role of the military in covert operations, which was made official policy in the Pentagon's Quadrennial Defense Review, published in February. Such activities, if conducted by C.I.A. operatives, would need a Presidential Finding and would have to be reported to key members of Congress.

" 'Force protection' is the new buzzword," the former senior intelligence official told me. He was referring to the Pentagon's position that clandestine activities that can be broadly classified as preparing the battlefield or protecting troops are military, not intelligence, operations, and are therefore not subject to congressional oversight. "The guys in the Joint Chiefs of Staff say there are a lot of uncertainties in Iran," he said. "We need to have more than what we had in Iraq. Now we have the green light to do everything we want."

The President's deep distrust of Ahmadinejad has strengthened his determination to confront Iran. This view has been reinforced by allegations that Ahmadinejad, who joined a special-forces brigade of the Revolutionary Guards in 1986, may have been involved in terrorist activities in the late eighties. (There are gaps in Ahmadinejad's official biography in this period.) Ahmadinejad has reportedly been connected to Imad Mughniyeh, a terrorist who has been implicated in the deadly bombings of the U.S. Embassy and the U.S. Marine barracks in Beirut, in 1983. Mughniyeh was then the security chief of Hezbollah; he remains on the F.B.I.'s list of most-wanted terrorists.

Robert Baer, who was a C.I.A. officer in the Middle East and elsewhere for two decades, told me that Ahmadinejad and his Revolutionary Guard colleagues in the Iranian government "are capable of making a bomb, hiding it, and launching it at Israel. They're apocalyptic Shiites. If you're sitting in Tel Aviv and you believe they've got nukes and missiles—you've got to take them out. These guys are nuts, and there's no reason to back off."

Under Ahmadinejad, the Revolutionary Guards have expanded their power base throughout the Iranian bureaucracy; by the end of January, they had replaced thousands of civil servants with their own members. One former senior United Nations official, who has extensive experience with Iran, depicted the turnover as "a white coup," with ominous implications for the West. "Professionals in the Foreign Ministry are out; others are waiting to be kicked out," he said. "We may be too late. These guys now believe that they are stronger than ever since the revolution." He said that, particularly in consideration of China's emergence as a superpower, Iran's attitude was "To hell with the West. You can do as much as you like."

Iran's supreme religious leader, Ayatollah Khamenei, is considered by many experts to be in a stronger position than Ahmadinejad. "Ahmadinejad is not in control," one European diplomat told me. "Power is diffuse in Iran. The Revolutionary Guards are among the key backers of the nuclear program, but, ultimately, I don't think they are in charge of it. The Supreme Leader has the casting vote on the nuclear program, and the Guards will not take action without his approval."

The Pentagon adviser on the war on terror said that "allowing Iran to have the bomb is not on the table. We cannot have nukes being sent downstream to a terror network. It's just too dangerous." He added, "The whole internal debate is on which way to go"—in terms of stopping the Iranian program. It is possible, the adviser said, that Iran will unilaterally renounce its nuclear plans—and forestall the American action. "God may smile on us, but I don't think so. The bottom line is that Iran cannot become a nuclear-weapons state. The problem is that the Iranians realize that only by becoming a nuclear state can they defend themselves against the U.S. Something bad is going to happen."

While almost no one disputes Iran's nuclear ambitions, there is intense debate over how soon it could get the bomb, and what to do about that. Robert Gallucci, a former government expert on nonproliferation who is now the dean of the School of Foreign Service at Georgetown, told me, "Based on what I know, Iran could be eight to ten years away" from developing a deliverable nuclear weapon. Gallucci added, "If they had a covert nuclear program and we could prove it, and we could not stop it by negotiation, diplomacy, or the threat of sanctions, I'd be in favor of taking it out. But if you do it"—bomb Iran—"without being able to show there's a secret program, you're in trouble."

Meir Dagan, the head of Mossad, Israel's intelligence agency, told the Knesset last December that "Iran is one to two years away, at the latest, from having enriched uranium. From that point, the completion of their nuclear weapon is simply a technical matter." In a conversation with me, a senior Israeli intelligence official talked about what he said was Iran's duplicity: "There are two parallel nuclear programs" inside Iran—the program declared to the I.A.E.A. and a separate operation, run by the military and the Revolutionary Guards. Israeli officials have repeatedly made this argument, but Israel has not produced public evidence to support it. Richard Armitage, the Deputy Secretary of State in Bush's first term, told me, "I think Iran has a secret nuclear-weapons program—I believe it, but I don't know it."

In recent months, the Pakistani government has given the U.S. new access to A. Q. Khan, the so-called father of the Pakistani atomic bomb. Khan, who is now living under house arrest in Islamabad, is accused of setting up a black market in nuclear materials; he made at least one clandestine visit to Tehran in the late nineteen-eighties. In the most recent interrogations, Khan has provided information on Iran's weapons design and its time line for building a bomb. "The picture is of 'unquestionable danger,' " the former senior intelligence official said. (The Pentagon adviser also confirmed that Khan has been "singing like a canary.") The concern, the former senior official said, is that "Khan has credibility problems. He is suggestible, and he's telling the neoconservatives what they want to hear"—or what might be useful to Pakistan's President, Pervez Musharraf, who is under pressure to assist Washington in the war on terror.

"I think Khan's leading us on," the former intelligence official said. "I don't know anybody who says, 'Here's the smoking gun.' But lights are beginning to blink. He's feeding us information on the time line, and targeting information is coming in from our own sources— sensors and the covert teams. The C.I.A., which was so burned by Iraqi W.M.D., is going to the Pentagon and the Vice-President's office saying, 'It's all new stuff.' People in the Administration are saying, 'We've got enough.' "

The Administration's case against Iran is compromised by its history of promoting false intelligence on Iraq's weapons of mass destruction. In a recent essay on the Foreign Policy Web site, entitled "Fool Me Twice," Joseph Cirincione, the director for nonproliferation at the Carnegie Endowment for International Peace, wrote, "The unfolding administration strategy appears to be an effort to repeat its successful campaign for the Iraq war." He noted several parallels:

The vice president of the United States gives a major speech focused on the threat from an oil-rich nation in the Middle East. The U.S. Secretary of State tells Congress that the same nation is our most serious global challenge. The Secretary of Defense calls that nation the leading supporter of global terrorism.