WASHINGTON — With the killing yesterday in Damascus of one of the deadliest leaders of modern Islamic terror, Imad Mugniyah, Israel may at long last have achieved the decisive victory that eluded it in the summer 2006 war against Hezbollah in Lebanon.

With the exception of Osama bin Laden and the late Abu Musab al-Zarqawi, no single enemy has more American blood on his hands. Mugniyah was the architect of the bombings of the American Embassy and Marine barracks in Beirut in 1983 and the 1985 hijacking of the TWA flight to Rome from Athens, during which the hijackers slew Robert Dean Stethem, throwing his body on the tarmac of the Beirut airport.

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Mugniyah was also suspected of a role in the 1992 bombing of Israel’s embassy in Buenos Aires and the 1994 bombing of the Jewish AMIA cultural center in Buenos Aires. More recently he was believed by analysts of the current war to have been an adviser to Iraq’s insurgency and the chief interlocutor between Iran and the Lebanon-based Hezbollah.

The notion that Israel’s special operation reach extends to Syria’s capital, coupled with the successful air strikes last September against an alleged Syrian nuclear facility, heightens the perception of Israeli power in the region, a view that had been attenuated by the 2006 conflict in Lebanon and by rocket strikes from Gaza on the Israeli city of Sderot. Israel officially denied participating in the killing.

The death of Mugniyah was hailed by the Bush administration and by both Democratic presidential candidates.

Senator Clinton issued a statement through her Senate office, saying, “Hezbollah leader Imad Mugniyah’s death in Syria is a reminder of the leading role he played in the deaths of hundreds of U.S. Marines in Lebanon and numerous other terrorist attacks. It offers an opportunity for us to honor his victims and remember the damage he inflicted on them and their families. We must continue to be vigilant against the threat posed by terrorist groups supported by Syria and Iran.”

Senator Obama said, “Imad Mugniyah earned a reputation as one of the world’s most brutal and notorious terrorists. He was responsible for the deaths of scores of Americans in heinous attacks. The world is now a safer place with him gone.”

The State Department spokesman, Sean McCormack, told reporters, “The world is a better place without this man in it. He was a cold-blooded killer, a mass murderer and terrorist responsible for countless innocent lives lost.”

America and Israel are preparing for the prospect of a Hezbollah counterstrike. Hezbollah’s satellite station, al-Manar, aired an interview with Hezbollah sheiks promising revenge.

The spokesman for Iran’s foreign ministry, Mohammed Hosseini, said, “This action is yet another brazen example of organized state terrorism by the Zionist regime.” An official statement from Hezbollah issued Wednesday said in part, “After a life full of Jihad, sacrifices and accomplishments lived with a longing to martyrdom, Islamic Resistance leader Hajj Imad Moghniyeh (Hajj Radwan) was assassinated by ‘Israeli’ criminal hands.” Also, Hamas spokesman, Sami Abu-Zuhr said, “We condemn this crime, and stress that the Muslim nation must rise and confront the Zionist Satan that is backed by the Americans.”

The Syrian interior minister, General Bassam Abdul-Majid, confirmed that Mugniyah was dead but refrained from pointing fingers.

Officially, Israel offered a soft denial. A statement from the prime minister’s office said, “Israel rejects the attempts of terror elements to attribute to Israel any involvement in this incident.” Meanwhile, a former deputy head of Israel’s Shin Bet domestic security service, Gideon Ezra, told Israel Radio, that many countries had an interest in killing Mugniyah and that “Israel too was hurt by him, more than other countries in recent years.” He added, “Of course I don’t know who killed him, but whoever did should be congratulated.”

Mugniyah was on the FBI’s list of most wanted international terrorists. A $25 million bounty on his head was later reduced to $5 million. Secretary of State Powell’s deputy, Richard Armitage, in 2003 referred to Hezbollah as the “A team” of terrorists.

In 1994, Israeli forces managed to kill Mugniyah’s brother, but Mugniyah himself for years remained elusive. He was said to divide his time among southern Lebanon, Damascus, and Tehran, and to have undergone plastic surgery to change his appearance and. Mugniyah’s demise comes as a blow to Hezbollah, Iran, and Syria. His death catches Hezbollah as it is embroiled in a power struggle in Lebanon and suspected by its confessional rivals of having been complicit in a spate of murders of anti-Syrian politicians.

His murder is also a blow to Syria’s president, Bashar al-Assad, who — unless he was complicit in the bombing — will face new questions about his efficacy because his ally was killed in his own capital, four months after Israeli jets destroyed an alleged nuclear facility in his territory. For Iran, Mugniyah was its chief bridge with southern Lebanon’s Hezbollah movement and one of its most skilled terrorist operatives.

The founder of the counterterrorism center the Central Intelligence Agency created in 1985 in part to find and capture Mugniyah, Duane Clarridge, said in an interview, “My view is this is long overdue. He was a psychopath. I was close to people who died at his hands in the Beirut Embassy bombing.”

One such person was William Buckley, the CIA station chief who was kidnapped, tortured, and eventually murdered in 1984. Mr. Clarridge today said Mugniyah’s role in murdering Buckley and the reports that Buckley was tortured and forced to eat rat poison made it personal for many CIA officers assigned to finding him.

Mr. Clarridge said he was impressed with the tradecraft of whoever pulled off the assassination yesterday. “What really flabbergasted me, is that this happened in Damascus. The bomb was in his car. That shows a level of access and intelligence that is rather superb,” he said.

Mr. Clarridge said the assassination suggests some level of cooperation from authorities in Syria, a police state. “This almost suggests possible local connivance. The Israelis certainly wanted to do it, but that is a capability that might be a bridge too far. Not that they didn’t want to. The Christians, elements of the Lebanese Christians have very good relationships in Syria and probably have some on-the-ground capability. They could well have done it for the Israelis, but they might have their own reasons.”

February 14 is the third anniversary of the assassination of a former Lebanese prime minister, Rafik Hariri, who was a leading politician pressing for the end of Syria’s occupation when he was killed in a car bomb at Beirut. The Syrian regime currently faces a U.N. tribunal for its role in that murder. A public funeral is also scheduled today for Mugniyah in the Hezbollah suburb of Dahiyeh.

A Lebanese-Christian-born analyst today said it was unlikely any Lebanese faction was involved in the Mugniyah killing.

“To say that any faction in Lebanon is behind this is to greatly misstate reality. They don’t have the operational capacity. They don’t have the intelligence capacity. It is extremely unlikely that anyone in Lebanon has anything to do with this,” a research fellow at the Foundation for the Defense of Democracies, Anthony Badran, said.