More than a decade before the international agency that monitors chemical weapons won the Nobel Peace Prize, John R. Bolton marched into the office of its boss to inform him that he would be fired.

“He told me I had 24 hours to resign,” said José Bustani, who was director general of the agency, the Organization for the Prohibition of Chemical Weapons in The Hague. “And if I didn’t I would have to face the consequences.”

Mr. Bolton, then an under secretary of state and later the American ambassador to the United Nations, told Mr. Bustani that the Bush administration was unhappy with his management style.

But Mr. Bustani, 68, who had been re-elected unanimously just 11 months earlier, refused, and weeks later, on April 22, 2002, he was ousted in a special session of the 145-nation chemical weapons watchdog.

The story behind his ouster has been the subject of interpretation and speculation for years, and Mr. Bustani, a Brazilian diplomat, has kept a low profile since then. But with the agency thrust into the spotlight with news of the Nobel Prize last week, Mr. Bustani agreed to discuss what he said was the real reason: the Bush administration’s fear that chemical weapons inspections in Iraq would conflict with Washington’s rationale for invading it. Several officials involved in the events, some speaking publicly about them for the first time, confirmed his account.

Mr. Bolton insists that Mr. Bustani was ousted for incompetence. In a telephone interview on Friday, he confirmed that he had confronted Mr. Bustani. “I told him if he left voluntarily we would give him a gracious and dignified exit,” he said.

As Mr. Bustani tells the story, the campaign against him began in late 2001, after Iraq and Libya had indicated that they wanted to join the Chemical Weapons Convention, the international treaty that the watchdog agency oversees. To join, countries have to provide a list of stockpiles and agree to the inspection and destruction of weapons, as Syria did last month after applying. Inspectors from the agency were making plans to visit Iraq in late January 2002, he said.

“We had a lot of discussions because we knew it would be difficult,” Mr. Bustani, who is now Brazil’s ambassador to France, said Friday in his embassy office in Paris. The plans, which he had conveyed to a number of countries, “caused an uproar in Washington,” he said. Soon, he was receiving warnings from American and other diplomats.

“By the end of December 2001, it became evident that the Americans were serious about getting rid of me,” he said. “People were telling me, ‘They want your head.’ ”

Mr. Bolton called on Mr. Bustani a second time. “I tried to persuade him not to put the organization through the vote,” Mr. Bolton said.

But still Mr. Bustani refused, and his fate was sealed. The United States had marshaled its allies, and at an extraordinary session, Mr. Bustani was ousted by a vote of 48 to 7, with 43 abstentions. He was reportedly the first head of an international organization to be pushed out of office this way, and some diplomats said the pressure campaign had made them uneasy.

Mr. Bolton’s office had also circulated a document that accused Mr. Bustani of abrasive conduct and taking “ill-considered initiatives” without consulting with the United States and other member nations, diplomats said.

But Mr. Bustani and some senior officials, both in Brazil and the United States, say Washington acted because it believed that the organization under Mr. Bustani threatened to become an obstacle to the administration’s plans to invade Iraq. As justification, Washington was claiming that Saddam Hussein, the Iraqi leader, possessed chemical weapons, but Mr. Bustani said his own experts had told him that those weapons were destroyed in the 1990s, after the Persian Gulf war.

“Everybody knew there weren’t any,” he said. “An inspection would make it obvious there were no weapons to destroy. This would completely nullify the decision to invade.”

Mr. Bolton disputed that account. “He made that argument after we invaded,” he said. Twice during the interview, Mr. Bolton said, “The kind of person who believes that argument is the kind who puts tin foil on his ears to ward off cosmic waves.”

But diplomats in The Hague said officials in Washington had circulated a document saying that the chemical weapons watchdog under Mr. Bustani was seeking an “inappropriate role in Iraq,” which was really a matter for the United Nations Security Council.