In the city that gave birth to modern suicide bombing attacks, the people of Beirut shouldn't have been surprised by today's assault on the Iranian embassy. But coming as it did on a target deep in the heart of Hezbollah-controlled South Beirut, all Lebanese will be shocked and very apprehensive.

It was Hezbollah, the militant Shia movement, that carried out many of the first suicide bombing attacks in Lebanon in the 1980s, and they will not suffer this attack on their Iranian ally lightly.

It was a Lebanese Sunni group, the local chapter of the al-Qaeda-linked Abdullah Azzam Brigades, that claimed responsibility for the deadly blasts, and it will be Lebanese that will suffer the wrath of Hezbollah now.

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Already Lebanon has endured outbreaks of violence between Sunni and Alawite groups that support different sides in the Syrian civil war. But these exchanges have mostly occurred in the North of the country where loyalties are most acutely divided.

The Iranian embassy attack, however, involves two principal groups fighting in the Syrian conflict. Hezbollah has been one of the most reliable battle-hardened forces in Syria helping prop up the regime of Bashar al-Assad, while the Abdullah Azzam Brigades are part of the opposition forces that still are waging war on the Iranian-backed regime in Damascus.

This suicide mission in Beirut has brought the frontline of the Syrian war deep inside Lebanon.