He infuriated Washington by challenging claims Saddam Hussein had a secret nuclear program, grappled with Iranian and North Korean nuclear programs, and brought luster and unprecedented scrutiny to his organization by winning the Nobel Peace Prize in 2005.
Mohamed ElBaradei, the outgoing chief of the International Atomic Energy Agency, is leaving behind a turbulent _ and controversial _ 12-year legacy as the public face of world diplomacy on keeping nuclear weapons out of the hands of rogue states.
In parting comments to his staff last week, ElBaradei said he was grateful "to be leaving at a moment when the agency has reached such prominence in contributing to international security and development."
But as he hands over to Yukiya Amano of Japan on Tuesday, most of the issues that threw the spotlight on him and the IAEA remain unresolved _ and of deep concern.
North Korea, which renounced the IAEA-monitored Nuclear Nonproliferation Treaty in 2002 and then quit the agency, exploded its second nuclear test weapon earlier this year. Syria is stonewalling IAEA attempts to probe U.S. and Israeli intelligence that it had a secret nuclear reactor geared to producing plutonium. And Iran has shrugged off three sets of U.N. Security Council sanctions to continue developing its once-secret nuclear program, despite fears it could be used to make weapons.
ElBaradei himself remains a figure of some controversy, praised by many antagonists of the United States for his willingness to stand up to the superpower.
For Norma Goichochea Estenoz, the chief IAEA delegate of Cuba, ElBaradei was an independent enforcer of the IAEA's mandate with an ear for the concerns of all. Iran's chief delegate, Ali Asghar Soltanieh, praises his backbone despite "tremendous political pressure" from the West.
But that independent streak was seen by the U.S. as being soft on Iran _ and led to attempts by Washington to have him removed from office.
The push was abandoned just before ElBaradei won the Nobel Peace Prize but it left the IAEA chief even more critical of the U.S, at least until the change of White House leadership last year.
"The adversarial relationship ... was not productive," notes William H. Tobey, a senior nonproliferation official in the U.S. Department of Energy until earlier this year, suggesting that the dispute ultimately worked to the benefit of Iran in its efforts to weaken international efforts to counter its nuclear defiance.
Much of the tension stemmed from the 2003 U.S.-led invasion of Iraq.
ElBaradei challenged Washington's assertion that Saddam Hussein had to be ousted because of a secret nuclear weapons program _ and no evidence of such activity has ever been found. He then invoked Iraq in arguing against the U.S.-led push to harshly punish Tehran, noting there was no proof for American assertions that the Islamic Republic had hidden nuclear weapons aspirations.
At the same time, he was tough on North Korea, accusing it of "nuclear brinkmanship" in December 2002 after it expelled two inspectors who were monitoring a mothballed nuclear complex.
"I'm calling a spade a spade," he told The Associated Press at the time. "I see a very serious crisis _ a country that's completely defying the world."
A tangible success in his push to curb nuclear proliferation came Friday, just three days before the end of his third term, when the IAEA's governing board agreed to establish an international nuclear fuel bank under agency oversight.
Such a project is meant to guarantee all nations supplies of enriched uranium from a neutral provider for their nuclear fuel needs _ and to cast suspicion on any country spurning that option in favor of its own enrichment program.
On Iran, he moved closer to the White House view late in his tenure, a shift made easier by the Obama administration's decision to stop isolating the Islamic Republic and negotiate nuclear differences _ attempts rebuffed by Tehran despite ElBaradei's activist role in making talks happen.
Addressing his last IAEA board meeting last week, he bluntly told the 35-nation gathering that his probe of Iran's nuclear program is at "a dead end" and that trust in Tehran's credibility is shrinking after its belated revelation that it was secretly building a nuclear facility.
The 67-year-old ElBaradei earned a bachelor's degree in law in 1962 at the University of Cairo. After a stint in the Egyptian Ministry of Foreign Affairs, he received a doctorate in International Law at the New York University School of Law in 1974, and later became an adjunct professor there before moving to the Vienna-based agency.
ElBaradei was appointed IAEA director general in 1997, heading what was back then a relatively obscure organization within the U.N. chain. Then came North Korea, and Iraq _ and ElBaradei gained stature under the intense media scrutiny. The man who once stumbled before television cameras started to speak in sound bites, growing ever bolder in his statements.
Reporters who covered ElBaradei over the next few years on missions to Tehran and elsewhere recall him swapping his suit jacket for a dark blue woolen sweater once his flight was in the air and then inviting them to the seat next to him for interviews.
ElBaradei freely exchanged jokes and anecdotes back then. But after his Nobel Prize win he became progressively remote.
"He was essentially unapproachable to all but a handful of closest advisers," said one agency official of ElBaradei's final years. He demanded anonymity because of the sensitivity of his comments.
ElBaradei himself told the New York Times he viewed himself as a "secular pope" whose mission is to "make sure, frankly, that we do not end up killing each other."
ElBaradei, who describes himself as having a Muslim background, sometimes cites his favorite Christian prayer when speaking of his role on the world stage. Attributed to St. Francis of Assisi, it begins: "Lord, make me an instrument of your peace."
In the last months of his term, the U.S. and its Western allies publicly lined up in praise of the austere Egyptian former diplomat. They endorsed a September IAEA resolution paying "tribute" to Elbaradei and lauding the "significant contribution" he has made to the work of the agency and "the cause of international peace and security during his distinguished and successful tenure as Director General."
Glyn Davies, the chief U.S. delegate to the, IAEA on Monday praised ElBaradei for his efforts in drawing up a deal that would have provided Iran with fuel for its Tehran research reactor from the Islamic Republic's stockpile of enriched uranium. World powers backed the deal _ which would have delayed Iran's ability to make a nuclear weapon by stripping it of most of its enriched material _ but Iran to date has not accepted it.
"Often controversial, sometimes exasperating, Director General Mohamed ElBaradei was always, in all things, a committed man of peace," Davies told The Associated Press.
But senior former U.S. officials make no secret of their view that he overreached on Tehran.
"I think he gave Iran encouragement and political cover, so I think he made the situation worse," says John Bolton, who served as Washington's ambassador to the U.N and as undersecretary of state in charge of the Iran nuclear file.
And through his refusal to declare Tehran's covert activities in violation of the Nuclear Nonproliferation Treaty, Tobey, the other former U.S. official, says ElBaradei "ultimately undermined U.N. Security Council efforts" to contain Iran's nuclear program.
To his last days in office, ElBaradei insisted he is above the fray, telling reporters last week that his engagement on Iran was "not a personal issue" but "an issue of security and of peace."
But he did occasionally get personal _ sometimes using his IAEA clout on issues that had nothing to do with the IAEA's mandate.
He canceled interviews with the British Broadcasting Corp. earlier this year over its refusal to air an appeal for victims of the Gaza conflict, asserting the decision violated "basic human decency."
Typically, say those who attended, his parting comments Friday to the staff over glasses of sparkling wine were inspirational _ even if they might be considered off the IAEA's message by critics.
"Carry forward the torch!" a visibly moved ElBaradei told the gathering, urging them to sustain their "100 percent commitment" to preserving and expanding peace, freedom, justice, and human dignity.