John D. Negroponte, the U.S. ambassador to the United Nations, knows a thing or two about regime change. As a young Foreign Service officer during the Vietnam War and later as ambassador to Honduras, he was at the center of troubled American efforts to overthrow communist governments in North Vietnam and Nicaragua.

But today, as the Bush administration weighs whether to forcibly disarm and topple Iraq's president, Saddam Hussein, Negroponte is emerging as a voice of caution.

"It's not for me to decide. It's the president's decision; he's our commander in chief," Negroponte said. "Obviously, we must be prepared to use force if necessary. But if you're asking me my view based on the most important experience I have had with regard to the use of force, which was Vietnam, it is one of caution."

Since he was sworn in as the top U.S. envoy to the United Nations on Sept. 18, 2001, just days after the Sept. 11 terror attacks against the United States, the 63-year-old career diplomat has found himself on the front lines of the war on terrorism and a key player in Washington's diplomatic effort to avert a war with Iraq. His chief priority is to maintain international pressure on Iraq to disarm under U.N. supervision while tightening import restrictions that would deny Baghdad the tools to battle a U.S.-led coalition.

"Regime change is really not something that's ever been dealt with in Security Council resolutions," he said. "It's not part of the purview of our U.N. policies."

Negroponte has prosecuted his task with an old-world civility that contrasts starkly with such Bush administration hawks as Defense Secretary Donald H. Rumsfeld and Vice President Cheney, who have derided the United Nations for its inability to uncover secret Iraqi arms caches. He defended the U.N. inspectors against charges emanating from Washington that they are not up to the task of disarming Iraq. He credited Hans Blix, the chief weapons inspector, with getting the "inspections started quicker than anybody expected." Even his dealings with Iraq have been polite. One of his first major assignments at the United Nations was to deliver a stern warning to Iraq's U.N. ambassador, Mohammed Douri, to advise his government to desist from any hostile acts against the United States or its allies while their troops were preoccupied with a war in Afghanistan or face military retaliation.

Negroponte cringed at media portrayals of himself at the time barging unannounced into the Iraqi mission on Oct. 7, the opening night of the U.S. bombing campaign in Afghanistan, to deliver the ultimatum.

"I didn't show up unannounced. I made an appointment," Negroponte said. "I am a rather correct and fairly old-fashioned diplomat and I don't think I would ever show up in anybody's mission without an appointment."

Diplomats here say Negroponte's cordial demeanor, while appreciated, masks a U.S. policy that is often anything but cordial. The United States threatened in June to shut down peacekeeping missions around the world to shield American soldiers from the reach of the International Criminal Court. In August, it warned the Security Council that the council would be condemned to irrelevance if it failed to back a tough resolution requiring Iraq's disarmament. And just last month, the United States broke with its closest allies when Negroponte cast the lone veto to block a Syrian-sponsored resolution condemning Israel for killing three U.N. aid workers.

The son of a Greek shipping magnate, Negroponte was born in London and raised at his family's Park Avenue home. Negroponte and his wife, Diana, adopted five children from Honduras.

A graduate of Yale University, he joined the Foreign Service in 1960 and rose quickly, advising Henry Kissinger during the Paris peace talks that ended the Vietnam War and later receiving senior posts at the State Department and White House. His former boss, Secretary of State Colin L. Powell, who was then national security adviser to President Ronald Reagan, persuaded him to leave the private sector at McGraw-Hill Cos. to take on the U.N. post.

It was Negroponte's stint in the early 1980s as ambassador to Honduras, where the Reagan administration was running a covert war against the Sandinista government in Nicaragua, that has attracted the most public scrutiny and contributed to delaying his Senate confirmation by more than six months.

At his confirmation hearing, Negroponte defended himself against charges that he had suppressed reports of human rights abuses by the Honduran military during his tenure. Citing the political turmoil plaguing Central America at the time, he appealed to the Senate not to judge his performance or the broader events in Honduras "through the exclusive prism of human rights considerations."

In the aftermath of Sept. 11, the Senate agreed, swiftly confirming him as President Bush's point man at the United Nations. Negroponte said his "plate has been full" since he arrived in New York, citing the struggle to manage crises involving al Qaeda, Iraq and the Middle East while paying courtesy calls to more than 150 foreign delegates. His only extended break came Sept. 21, when he checked into the hospital for prostate cancer surgery in the middle of the U.S. effort to pass the resolution on Iraq's disarmament. He was back at work within 10 days.

"Secretary Powell had actually asked me if I could postpone the operation. I talked to my surgeon and he recommended against it," Negroponte said. Less than two months after his operation, Negroponte helped negotiate a deal that united the Security Council behind a tough U.S.-sponsored resolution that provided Iraq a "final opportunity" to disarm under U.N. supervision or face a possible military attack. He credited Bush and Powell with providing the political muscle required to close the deal.

"This was very much of a collective effort from the president on down," Negroponte said. "That led me to conclude that the prospects for a successful outcome had to be pretty good. But it doesn't mean to say it was easy."

Ambassador John D. Negroponte was sworn in to his U.N. post days after the Sept. 11, 2001 attacks.