A year ago, as U.S. troops swept toward Baghdad, Ahmed Chalabi and about 400 hastily assembled fighters were secretly airlifted into southern Iraq to rally other Iraqis and begin a march toward Baghdad to help topple Saddam Hussein, an operation that won the concurrence of U.S. officials all the way up to Vice President Cheney's office. Chalabi had predicted that he would become Iraq's Spartacus, cutting a wide swath through Iraq and mobilizing vast numbers behind him, according to U.S. officials.

It marked the high point in Chalabi's checkered career -- and in his relationship with the United States. But it did not last long.

"It was the moment of truth for Chalabi, and it was literally a moment. It was over almost the minute it happened," said a senior U.S. official who worked with Chalabi and served in the U.S.-led coalition in Baghdad. "Compared to [Charles] de Gaulle's march to Paris [to liberate France], Chalabi's march to Baghdad was a stone that went into the water without a splash."

U.S. officials point to that early April 2003 covert operation as the turning point in their dealings with the charismatic U.S.-educated banker and convicted felon -- a relationship that was always controversial but, nonetheless, has dramatically changed both Iraq and the Middle East over the past year.

Instead of being the warrior-king who liberated town after town, "he was jeered more than cheered. Iraqis were shouting him down. It was embarrassing," said another U.S. official familiar with Chalabi's first public appearance in the Iraqi heartland after 45 years in exile. "We had to help bail him out."

Since then, Chalabi's standing has gradually eroded, until yesterday, when a U.S.-backed raid on his compound marked a new nadir. Although Chalabi has always been a divisive figure, even quarters that once strongly supported him were distancing themselves yesterday. Many administration officials would not speak on the record yesterday because of the contentious relations some had with him.

"The vast majority of reports of his proximity to and influence on administration policy have been greatly exaggerated," said a senior administration official involved in Iraq policy who knows Chalabi. "The reality is that he was among a wide variety of Iraqi figures who made the case to an array of American officials over a period of time for the liberation of the Iraqi people."

Yet no Iraqi leader has had more to do with the U.S. intervention in Iraq than Chalabi, from charming Congress into authorizing almost $100 million to back his fledgling Iraqi National Congress in the late 1990s and convincing Washington about Hussein's weapons of mass destruction in 2002 to pressing for war last year, say both his supporters and critics.

"He was impressive in Washington because, amongst a feckless crew of Iraqi exiles, he was the one who stood out for his intelligence and organizational abilities and his courage. But Ahmed has a fatal flaw. He's too clever by half, and he's challenged by the truth, which has been the repeated pattern of his undoing," said Martin S. Indyk, a former assistant secretary of state and National Security Council staff director who dealt with Chalabi when the Clinton administration was exploring how to oust Hussein.

When he arrived in Baghdad, after U.S. troops liberated the Iraqi capital, Chalabi almost immediately began rubbing U.S. officials the wrong way by asserting himself -- and becoming a rival authority, U.S. officials say.

One of his aides declared himself "mayor" of Baghdad. His supporters established what U.S. officials called "Chalabi cantons," complete with roadblocks and tolls. And loyalists sent out word that Iraqis should report to the Iraqi National Congress (INC) before returning to work.

His agents were also faster than U.S. troops at getting to Iraq's intelligence headquarters, where they took thousands of sensitive files, which the INC has refused to return to the new intelligence ministry, U.S. officials say. Supporters were implicated in commandeering the property of former Baath Party officials, from homes to upscale cars.

But with strong support from Washington, Chalabi continued to play an important role in Iraq as the U.S.-led coalition grappled with finding local leaders in an effort to shape postwar Iraq.

"He won the confidence of the neo-conservatives, plugged into their wavelength and articulated a vision that was identical to the one they had. What he said about Baathism, weapons of mass destruction, terrorism, Saddam and the future of the Middle East was indistinguishable from what they believed," the senior U.S. official said.

Chalabi was among the 25 Iraqis selected last summer for the Iraqi Governing Council, but relations continued to fray. The U.S. failure to find weapons of mass destruction during the summer and fall further undermined his credibility -- and irritated the Bush administration. INC intelligence and defectors played a major role in building the case against Hussein, U.S. officials say.

"Now it's demonstrable that he told the U.S. government a lot of things that were not true," said Pat Lang, former head of Middle East intelligence at the Defense Intelligence Agency. At the United Nations last year, Secretary of State Colin L. Powell presented the U.S. case for war, which included information on mobile labs for the production of chemical or biological weapons based on data from a defector provided by the INC, data that the United States has since conceded were untrue.

But Chalabi, a Shiite Muslim educated at MIT and the University of Chicago, has been unrepentant. "We are heroes in error," he told the Daily Telegraph of London in February. "As far as we're concerned, we've been entirely successful. That tyrant Saddam is gone and the Americans are in Baghdad. What was said before is not important."

Throughout much of its relationship with him, the United States has been willing to shrug off Chalabi's past as a convicted felon disliked and mistrusted by many in the Arab world. In 1992, Chalabi, whose family fled Iraq when he was a teenager, was sentenced in absentia by Jordan to 22 years in prison on 31 counts of embezzlement and other bank fraud charges.

But Chalabi's close relationship with Iran, the only neighboring state that regularly deals with him, is now a further cause of concern in Washington. The INC chief has always been a master at balancing the two foes, but U.S. officials have recently cited fears that Chalabi's ties could endanger U.S. operations in Iraq.

As U.S. and U.N. officials work to form an interim Iraqi government, U.S. officials have increasingly been frustrated by Chalabi's maneuvering to ensure that he and some of his Governing Council allies retain strong positions. Washington fears that he will try to undermine whomever U.N. envoy Lakhdar Brahimi names, possibly next week.

Chalabi insisted yesterday that he is still "America's best friend in Iraq," although he later told reporters that he is severing ties with the U.S.-led coalition government and now wants to see Iraq liberated. "Let my people go," he said. "It is time for the Iraqi people to run their affairs."

Researcher Julie Tate contributed to this report.