In a top-secret adjunct to an openly reported diplomatic initiative, U.S. and allied intelligence services summoned scores of Iraqi operatives in foreign capitals to present a stark choice. They were told "they could either `turn,"' said one official, using an expression for switching sides, or be expelled back to Iraq "to enjoy your very short stay in Baghdad."

Another official with access to written accounts of the conversations said the Iraqis were told that when the United States sorts friends and enemies after toppling President Saddam Hussein, "they'll be putting themselves and their families at the mercy of the new Iraqi government."

The State Department announced on March 6 that it had asked 60 friendly governments to expel alleged Iraqi intelligence operatives who lived abroad under diplomatic or commercial cover. Spokesman Philip Reeker portrayed the request as routine. But behind the announcement was Operation Imminent Horizon, in which Iraqis were pressured to provide information about the weapons programs and Iraqi operational plans. Among the nations who helped with the expulsions and recruiting efforts were Romania, Hungary, Australia and Sweden, officials said.

The Defense Department is racing to integrate the new leads into an extremely risky and ambitious disarmament mission. The quality of intelligence on Iraqi chemical, biological or nuclear weapons could not only determine the threats facing U.S. troops on the battlefield in the days ahead, but also could become a factor in conclusions around the world about whether the war was necessary.U.S. planners are urgently focused on the speedy capture of Iraqi scientists and identification of suspected weapons sites, to prevent attacks on U.S. forces and preserve evidence of proscribed programs. But they are also wary of booby traps and the possibility that small U.S. disarmament teams could be overwhelmed if they outrun friendly ground forces.

Officials said many U.S. analysts feared that the chaos of the Iraqi government's collapse could give an opening to Iraqi scientists or security officials inclined to seize and sell special weapons for profit or revenge. That would bring about the very proliferation that the war is intended to prevent.

One conundrum facing U.S. officials is the need for early and concrete proof of an ongoing Iraqi special weapons program even with the likelihood, as some officials see it, that the program will not be fully understood for months or years.

"A very important political component is if you find these things, how do you establish the proof of that to the satisfaction of 35 foreign ministries and those of you in the media?" said Jay Davis, who led the Defense Threat Reduction Agency until 2001 and has continued to consult on the Iraqi disarmament plan. "A large number of conspiracy theorists all over the world will say the U.S. government has planted all that stuff."

In a twist that could complicate the effort, the White House has decided, for now, to assign no role in the disarmament hunt to the key U.N. agencies that were charged by the Security Council with carrying out the search for banned weaponsAs recently as the last week of February, U.S. and U.N. officials said, the State Department asked the leading inspection agency, the United Nations Monitoring Verification and Inspection Commission, known as UNMOVIC, and the International Atomic Energy Agency for help in the postwar dismantling of Iraq's remaining long-range missiles and nuclear, biological, chemical and missile weapons programs.

But early this month, the White House reversed course, saying it was a military operation and would remain one for a long time to come, not suitable for civilians. Officials said that the United Nations agencies could be given a role later. Two officials said the U.N. agencies would not likely be invited to participate until the United States was ready to turn over dual-use biological or chemical sites for long term monitoring. U.N. sources said the IAEA believes it has ongoing legal authority over former Iraqi nuclear facilities, regardless of a change in government.

As substitute for the U.N. expertise, the Bush administration has scrambled to recruit former inspectors from the United Nations Special Commission, or UNSCOM, the agency that carried out inspections in Iraq in the 1990s. These inspectors may join military-civilian "exploitation teams" that will interrogate Iraqi weapons scientists and assess captured documents, but the first of them are just arriving in the region, and their roles remain undefined.

Charles Duelfer, who served as deputy executive chairman at UNSCOM for most of its eight-year life, departed for Kuwait on Monday. Duelfer was highly regarded among disarmament experts, but UNSCOM's back channels to Washington and London aroused suspicion among other nations on the U.N. Security Council. UNSCOM dissolved in controvery in 1999 over disclosures that the United States had used the commission's resources to evesdrop on the Iraqi military. Duelfer left no forwarding telephone number and did not reply to e-mail sent Tuesday to the address he supplied on his office voicemail.

Since there will be no independent U.N. witnesses in the search, U.S. officials said they give high priority to making a public display of the discovery of any weapons. "If we've got a good news story, in the sense that we find something, we will take reporters to see it," one defense official said. Another spoke of a mandate to find "a smoking gun real fast." He added: "And if the son of a bitch uses a couple of chemical artillery shells, he will have done the same thing."

Participants in the U.S. disarmament planning, most of them speaking this week without official permission and on condition of anonymity, expressed disquiet about the prospects of success. The venture, said one official who echoed other colleagues, remains improvised and unfinished, "with about 74,000 moving pieces." Few of those interviewed believed they could predict the performance of newly invented task forces that fuse civilian experts with intelligence, Energy Department and Justice Department employees ­ all under military operational command. Some sources said the uncertainties reflect the relative inattention of top policymakers, who turned to disarmament only weeks ago after more than a year of concerted planning to remove the Iraqi government by force.

A senior defense official with responsibility for oversight of the mission, speaking for the Pentagon but insisting on anonymity, expressed pride in the on-the-fly assembly of experts but acknowledged they begin their work amid great uncertainty.

"It's going to be very fluid," the official said. "Things might not be where we think they are. We may stumble onto things we didn't know about."

Officals said the United States has more information on President Saddam Hussein's weapons programs than it has shared publicly or with U.N. inspectors. They said they withheld information only to preserve essential sources, methods or the secrecy of ongoing operations. But success or failure of the U.S. disarmament plan depends heavily on extracting more details from a group of 75 to 100 Iraqis with primary knowledge of Iraq's weapons design, procurement, manufacture and concealment.

Special Forces teams are poised to conduct raids from the earliest hours of the war to seize targets of opportunity among those men and women. U.S. intelligence is tracking their whereabouts, and in some cases "we have a pretty good idea where they are," one official said.

A handful of eleventh-hour recruits have come from among Iraqi scientists abroad who have also been approached lately by U.S. agents. The Baghdad government sent some of them overseas and elsewhere late last year to prevent their questioning by U.N. inspectors. Still other knowledgeable Iraqis have been detained at border points this month as they sought to flee the country before shooting started.

American experts are counting on many Iraqis to cooperate eagerly, but others, one official directly involved in the planning said, will be faced with "an interrogation that doesn't look anything like an interview with U.N. inspectors." The Iraqis can tell what they know, the official said, or face "handling as enemy combatants. They'll go to a war crimes tribunal with Saddam, or they will be in prison. You have a lot of leverage over them."

It was not clear, government officials said, whether all the central players in Iraq's weapons program would be offered amnesty for information. Asked whether any were regarded as war criminals or otherwise irredeemable enemies, five officials said the government had made no such decision.

Military planners see four stages in the disarmament campaign. The first is to take control of and assess any known site that might present an immediate threat to U.S. forces. The second is to disable the threat and any ongoing production. The third will be the responsibility of "exploitation teams," armed with tools to extract information from hidden or encrypted computer files, linguists and field laboratories that include detectors for radiation and sophisticated tests for biological and chemical toxins. Full destruction, the fourth stage, will come much later.

"We are not going to be blowing up munitions and destroying things if they do not pose an immediate threat," a Pentagon civilian planner said. "We will secure it and then come back, when we're in a permissive environment, to destroy the material in a way that's safe to civilians and soldiers."

The weapons hunt draws on nuclear experts from the Lawrence Livermore, Los Alamos and Sandia national laboratories, civilian scientists from the Energy Department's Nuclear Emergency Response Team, linguists from the CIA and Defense Intelligence Agency, and computer and records specialists from the Justice Department. The military is supplying specialists in missiles and biological and chemical weapons, drawn from DTRA, the Cooperative Threat Reduction Agency, the Army's Technical Escort Unit and the Marine Corps Chemical Biological Incident Response Force.

Some of those involved said they had been advised to think of their job as a long term commitment.

"I don't know how anyone can even begin to make an estimate of how long the war will be, how long this process will be," said one participant.