The Bush Administration, while publicly advocating diplomacy in order to stop Iran from pursuing a nuclear weapon, has increased clandestine activities inside Iran and intensified planning for a possible major air attack. Current and former American military and intelligence officials said that Air Force planning groups are drawing up lists of targets, and teams of American combat troops have been ordered into Iran, under cover, to collect targeting data and to establish contact with anti-government ethnic-minority groups. The officials say that President Bush is determined to deny the Iranian regime the opportunity to begin a pilot program, planned for this spring, to enrich uranium.

American and European intelligence agencies, and the International Atomic Energy Agency (I.A.E.A.), agree that Iran is intent on developing the capability to produce nuclear weapons. But there are widely differing estimates of how long that will take, and whether diplomacy, sanctions, or military action is the best way to prevent it. Iran insists that its research is for peaceful use only, in keeping with the Nuclear Non-Proliferation Treaty, and that it will not be delayed or deterred.

There is a growing conviction among members of the United States military, and in the international community, that President Bush’s ultimate goal in the nuclear confrontation with Iran is regime change. Iran’s President, Mahmoud Ahmadinejad, has challenged the reality of the Holocaust and said that Israel must be “wiped off the map.” Bush and others in the White House view him as a potential Adolf Hitler, a former senior intelligence official said. “That’s the name they’re using. They say, ‘Will Iran get a strategic weapon and threaten another world war?’ ”

A government consultant with close ties to the civilian leadership in the Pentagon said that Bush was “absolutely convinced that Iran is going to get the bomb” if it is not stopped. He said that the President believes that he must do “what no Democrat or Republican, if elected in the future, would have the courage to do,” and “that saving Iran is going to be his legacy.”

One former defense official, who still deals with sensitive issues for the Bush Administration, told me that the military planning was premised on a belief that “a sustained bombing campaign in Iran will humiliate the religious leadership and lead the public to rise up and overthrow the government.” He added, “I was shocked when I heard it, and asked myself, ‘What are they smoking?’ ”

The rationale for regime change was articulated in early March by Patrick Clawson, an Iran expert who is the deputy director for research at the Washington Institute for Near East Policy and who has been a supporter of President Bush. “So long as Iran has an Islamic republic, it will have a nuclear-weapons program, at least clandestinely,” Clawson told the Senate Foreign Relations Committee on March 2nd. “The key issue, therefore, is: How long will the present Iranian regime last?”

When I spoke to Clawson, he emphasized that “this Administration is putting a lot of effort into diplomacy.” However, he added, Iran had no choice other than to accede to America’s demands or face a military attack. Clawson said that he fears that Ahmadinejad “sees the West as wimps and thinks we will eventually cave in. We have to be ready to deal with Iran if the crisis escalates.” Clawson said that he would prefer to rely on sabotage and other clandestine activities, such as “industrial accidents.” But, he said, it would be prudent to prepare for a wider war, “given the way the Iranians are acting. This is not like planning to invade Quebec.”

One military planner told me that White House criticisms of Iran and the high tempo of planning and clandestine activities amount to a campaign of “coercion” aimed at Iran. “You have to be ready to go, and we’ll see how they respond,” the officer said. “You have to really show a threat in order to get Ahmadinejad to back down.” He added, “People think Bush has been focussed on Saddam Hussein since 9/11,” but, “in my view, if you had to name one nation that was his focus all the way along, it was Iran.” (In response to detailed requests for comment, the White House said that it would not comment on military planning but added, “As the President has indicated, we are pursuing a diplomatic solution”; the Defense Department also said that Iran was being dealt with through “diplomatic channels” but wouldn’t elaborate on that; the C.I.A. said that there were “inaccuracies” in this account but would not specify them.)

“This is much more than a nuclear issue,” one high-ranking diplomat told me in Vienna. “That’s just a rallying point, and there is still time to fix it. But the Administration believes it cannot be fixed unless they control the hearts and minds of Iran. The real issue is who is going to control the Middle East and its oil in the next ten years.”

A senior Pentagon adviser on the war on terror expressed a similar view. “This White House believes that the only way to solve the problem is to change the power structure in Iran, and that means war,” he said. The danger, he said, was that “it also reinforces the belief inside Iran that the only way to defend the country is to have a nuclear capability.” A military conflict that destabilized the region could also increase the risk of terror: “Hezbollah comes into play,” the adviser said, referring to the terror group that is considered one of the world’s most successful, and which is now a Lebanese political party with strong ties to Iran. “And here comes Al Qaeda.”

In recent weeks, the President has quietly initiated a series of talks on plans for Iran with a few key senators and members of Congress, including at least one Democrat. A senior member of the House Appropriations Committee, who did not take part in the meetings but has discussed their content with his colleagues, told me that there had been “no formal briefings,” because “they’re reluctant to brief the minority. They’re doing the Senate, somewhat selectively.”

The House member said that no one in the meetings “is really objecting” to the talk of war. “The people they’re briefing are the same ones who led the charge on Iraq. At most, questions are raised: How are you going to hit all the sites at once? How are you going to get deep enough?” (Iran is building facilities underground.) “There’s no pressure from Congress” not to take military action, the House member added. “The only political pressure is from the guys who want to do it.” Speaking of President Bush, the House member said, “The most worrisome thing is that this guy has a messianic vision.”

“The planning is enormous,” the former senior intelligence official said, referring to the activity at the U.S. Central Command headquarters, in Florida; the Joint Warfare Analysis Center, in Virginia; and the U.S. Strategic Command, in Nebraska. “Space assets, SLBMs”—submarine-launched ballistic missiles—“tactical air, and sabotage, coöperation from the Turks and the Russians.” He added that the plans include “significant air attacks on their countermeasures and anti-aircraft missiles—a huge takedown.” He depicted the planning as hectic, and far beyond the contingency work that is routinely done. “These are operational plans,” the former official said.