The Abdullah Azzam Brigades, a Sunni extremist group based in Palesitne and closely linked to al-Qaida, claimed responsibility for the attack -- and threatened further attacks if Hezbollah does not pull its forces out of Syria, where they're supporting the embattled regime of President Bashar Assad, Iran's key Arab ally who the predominantly Sunni Syrian rebels are fighting to destroy.
Hezbollah's stronghold in the Shiite-dominated Dahiyeh area of south Beirut has been bombed twice in recent weeks in what was seen as Sunni revenge for aiding Assad, and probably intended to provoke retaliation against Lebanon's Sunnis and open up a new front that would force Hezbollah to withdraw its fighters from Syria.
A car bomb wounded 50 people July 9 in the first serious attack on civilians in the Hezbollah bastion since Lebanon's 1975-90 civil war. A bigger car bomb killed 27 and wounded 300 Aug. 15.
Forty-seven Sunnis were slaughtered in the northern city of Tripoli Aug. 23 when two mosques were bombed. Lebanese authorities accuse Syrian intelligence of being responsible, aided by Lebanese Alawites, the minority sect that dominates the Syrian regime.
Hezbollah has not responded directly, although the pressure must have been considerable, but its leader, the charismatic Sheik Hassan Nasrallah, understands that fighting Sunni militants in Lebanon, affiliates of al-Qaida, would mean seriously weakening Assad's forces in Syria, where al-Qaida is now one of the main rebel players.
Security analysts and western intelligence sources in Beirut believe that even Tuesday's attack on the embassy, in which an Iranian official identified as the cultural attache was killed, is not likely to push Nasrallah into a new fight -- for now anyway.
It seems instructive then that Nasrallah, who has vowed to save the Syrian regime, made no public response to the carnage.
Instead, Ali Mikdad, a Hezbollah member of Lebanon's parliament, did the talking, indicating Hezbollah will stay its hand -- for now.
"We tell those who carried out the attack, you will not be able to break us," he declared. "We got the message and we know who sent it, and we know how to retaliate."
Iran needs Assad in power to ensure the land corridor Syria provides into Lebanon remains intact, to maintain arms supplies to Hezbollah, Tehran's forward military force against Israel.
The Syrian conflict and the Lebanese tensions are part of a wider, much more dangerous confrontation between Saudi Arabia, which considers itself the champion of Islam's mainstream Sunni sect, and Iran, the only Shiite-dominated state in the region, which is seen to have expansionist aims to challenge the Sunnis' 14 centuries of dominance in Islam.
Tuesday's attack came as a new Syrian regime offensive, aimed at seizing strategic rebel-held ground in the Qalamoun Mountains between Damascus and Lebanon's smoldering eastern border, gathered momentum.
The brewing battle, for which both sides have been preparing for weeks, could be the deciding factor in the 32-month-old war, with Hezbollah playing what could be its biggest role in the conflict so far. It's reported to have massed several thousand fighters to spearhead the operation, adding weight to the premise it will avenge the Beirut attacks at a time of its own choosing.
Observers fear the Saudis, alarmed at the prospect of a rapprochement between the United States and Iran that could leave Assad in power, may want to stir things up in Lebanon with more deadly bombings to draw off Hezbollah forces from Syria.
The Saudis have been involved in attacking Hezbollah in this manner before.
During Lebanon's civil war, Riyadh was behind a CIA-inspired March 8, 1985, car bomb attack outside a mosque in south Beirut, intended to assassinate a revered Shiite cleric, Sheik Mohammed Fadallah -- who was considered by the Americans, wrongly as it turned out, to be Hezbollah's spiritual head.
Fadlallah survived the attack, financed with funding arranged by then-Saudi Ambassador to the United States, Prince Bandar. But 83 people, mostly women and schoolgirls, were killed.
Bandar is now director of Saudi Arabia's main intelligence service, whose main mission is destroying Assad.