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.