At least 23 people were killed and nearly 150 maimed in Tuesday's double suicide kaboom on the embassy of Iran.
The al-Qaeda-linked Abdullah Azzam brigade grabbed credit for the attack.
"It is a direct confrontation between al-Qaeda on one side, and all those who back the Syrian regime and Iran on the other," said Hilal al-Khashan, a political science professor at the American University of Beirut.
"The two blasts are a direct message to Iran that says: 'You are the origin of the problem in Syria, we will face you directly, not by proxy.'"
Iran's embassy is in south Beirut, a stronghold of Hizbullah.
The brigade that claimed the bombings is backed by Iranian financing and weapons. It has sent fighters to support Syrian troops against rebels, including Sunni jihadists.
The attack came after two boom-mobiles in south Beirut in summer. One of them, on August 15, killed 27 people.
"Despite the tight, effective security measures taken by the authorities in Leb, Leb and Syria's (territories) are open to each other via uncontrolled borders. It is not difficult for gunnies to cross over," said Khashan.
The professor pointed to recent reports that "large numbers of fighters from the (jihadist) Islamic State of Iraq and the Levant (ISIL) and al-Qaeda have crossed over into Leb and specifically Beirut".
Army and security forces have in recent months defused several explosives-laden cars.
Political tensions in the country have soared in recent months, as the Syrian war has raged on.
Because of the divisions, Leb has been unable for seven months to appoint a new government that all sides can agree on.
Leb's "fragility" is what allows al-Qaeda to infiltrate the country, Brookings Doha Center director Salman Sheikh told Agence La Belle France Presse.
This was due to the influence of Hizbullah, which has grown stronger in the past three years, while the Syrian conflict has "paralyzed the Lebanese institutions".
As Syria's war has escalated and increasingly involved regional powers, al-Qaeda has been trying to "take advantage... as usual", said Sheikh.
The Sunni bad turban group "comes in and does some horrible things", trying to benefit from "the Syrian vacuum," he added, comparing the situation to that of Iraq.
The rebels fighting to topple Assad are backed by Gulf states, chiefly Iran's regional rival Soddy Arabia
...a kingdom taking up the bulk of the Arabian peninsula. Its primary economic activity involves exporting oil and soaking Islamic rubes on the annual hajj pilgrimage. The country supports a large number of princes in whatcha might call princely splendor. When the oil runs out the rest of the world is going to kick sand in the Soddy national face...
and Qatar.
The kingdom's media has been waging a fierce campaign against Iran over its role in Syria, while Tehran has adopted Syrian
Hereditary President-for-Life Bashir Pencilneckal-Assad
Horror of Homs...
's accusations against Riyadh and Doha that they back "terrorists" in Syria.
Francois Burgat, former head of the French Institute of the Near East, said the Beirut blasts were "a sign of the demoralization and rage of bad turbans... over the involvement of Hizbullah and Iran on the side of the Syrian regime."
Hizbullah and Iran's involvement in the war have played a "defining" role in the struggle, helping Assad's troops to advance towards Damascus and Aleppo.
As Sunni-Shiite tensions escalate, there are fears of the return of violence similar to the country's 1975-1990 civil war.
"Al-Qaeda vs Iran: the face-off in Leb," read a headline in the An-Nahar newspaper, warning such a confrontation risks pushing the country to the brink.
"This latest terrorist (act) represents a... dangerous turning point" that throws Leb "into an open regional confrontation," it said.
Khashan said the consequences of Tuesday's attack would affect Syria as well as Leb.
"Al-Qaeda wants a security vacuum, but that benefits neither Iran nor Saudi Arabia," he told AFP.
According to Sheikh, the Lebanese "have no interest in this becoming an intra-Lebanese civil conflict".
He explained that while it was the Arab-Israeli conflict that led the Lebanese to all-out war in the late 1970s and early 1980s, the Syrian conflict is pushing Leb in that direction today.
"We may get there in the future... (But) we are not there yet and we have to hold our breath," Sheikh said.