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.’ ”