Iraq is clearly first on the target list for such action, and already the Central Intelligence Agency and the Defense Department have stepped up efforts to unseat Saddam Hussein in a last effort to avoid the necessity of a full-scale invasion.
Yet the policy, a significant move away from the chesslike military strategies of the cold war, deals more broadly with a range of options to prevent nations from obtaining large-scale weapons or sponsoring terrorism. The strategy will probably be completed in August, when the president is here on vacation.
His aides said they are fine-tuning the policy to make it clear that the United States has options beyond armed intervention. Those options include joint operations with Russia and other powers. Potential targets include weak states that have become, in the words of one official, "petri dishes" for terrorist groups.
Mr. Bush emphasized pre-emption when he addressed the German Parliament last month. He expanded on the theme at West Point two weeks ago, saying, "If we wait for threats to fully materialize, we will have waited too long." On Friday, at a Republican fund-raiser, he called his approach a "new doctrine," although it echoes actions that past presidents have taken, notably President Kennedy's quarantine of Cuba during the 1962 missile crisis.
"It really means early action of some kind," Condoleezza Rice, the national security adviser, said in a recent interview. "It means forestalling certain destructive acts against you by an adversary." There are times, she said, when "you can't wait to be attacked to respond."
Although Ms. Rice describes the new policy as a broad one, and one that names no countries or terrorist groups, it is already being set in motion against Iraq.
Twice since Sept. 11, Mr. Bush has signed findings authorizing more spending for Iraqi opposition groups, with a focus on intelligence-gathering and on the infiltration by American Special Operations forces and C.I.A. operatives.
The latest order authorizes those forces to kill Mr. Hussein only in self defense, The Washington Post reported today, expanding on a report in USA Today on Feb. 28. But a senior administration official said today that the order made no reference to "targeting Saddam," and it would not waive the prohibition on assassinating a foreign leader.
"The problem with a full-scale invasion is that you lose the element of surprise, which is often critical in pre-emption," a senior official said today. "So the president wants to try everything short of that, because he knows that if we have to mount an invasion force, Saddam will see it coming."
Discussions within the White House have dwelled on examples that suggest that the most successful pre-emptive actions were not the most drastic military options. Ms. Rice noted that President Kennedy "thought about a lot of possibilities" during the 1962 missile crisis, but rejected advice to launch a direct attack on the Soviet missile sites being built on island.
"They settled on a strategy that actually was pre-emptive, but didn't use military force to do it, and thereby preserved the possibility for the Soviets to back down," she said. She would not apply the lesson to the current Iraq debate, but said that "there's a whole range of possible ways to take early action."
Others involved said that White House discussions have taken up other cases: President Johnson's consideration, for example, of a pre-emptive strike against China to prevent it from deploying nuclear weapons. The option was abandoned.
The drafters of the policy have also given thought to cases in which presidents failed to act pre-emptively, including not moving more actively against Nazi Germany in the 1930's.
With that in mind, administration officials and some outside consultants have discussed what kind of pre-emptive options Mr. Bush would have to choose from if intelligence concluded that a nation was about to obtain or export weapons of mass destruction. They have also considered how the United States should react if Islamic militants in Pakistan, for example, tried to seize the country's nuclear weapons.
In public statements, Mr. Bush has not discussed his policy in terms of individual countries or terrorist groups. But in private conversations with his close circle, it is is a near-constant source of discussion. Those officials include Secretary of State Colin L. Powell; Ms. Rice; her deputy for counterterrorism, Gen. Wayne Downing; Secretary of Defense Donald H. Rumsfeld; and other members of the national security team. He has also taken it up with members of Congress.
Referring to action against Iraq, Senator Joseph Biden, the Delaware Democrat who is chairman of the Foreign Relations Committee, said today on CBS, "I have discussed this with the President," and "at length" with Ms. Rice and other aides, and was convinced that in the campaign to oust Mr. Hussein, "there's a clear way to do this."
But recently, Mr. Bush has sprinkled his public statements with references to pre-emption as a pillar of a campaign against stateless terrorists or against states that would slip them nuclear or biological material.
In Berlin last month, Mr. Bush stood in the Reichstag, whose burning in 1933 marked the beginning of Hitler's rise as Europe stood by, and warned his European allies that "wishful thinking" would not eliminate "the new totalitarian threat."
The president's aides say the new policy rewrites the fundamental strategies that guided American thinking during the cold war.
The first of those strategies was containment the policy of living with the nuclear power of the Soviet Union but preventing its expansion. The second was deterrence, which assumed that America's defenses could be arrayed to assure a devastating response and therefore keep the enemy from acting.
Both strategies fit within the United Nations charter, which gives a nation a right to defend itself when attacked but offers little room for countries to define when they felt threatened.