It threatens to reignite the bitter 15-year Lebanese civil war at a time when the country is overwhelmed with more than 800,000 Syrian refugees - about a fifth of its own population.The attack, claimed by an al-Qa'ida offshoot, is clear revenge for Iran's support for President Bashar al-Assad and its backing for Hezbollah, the Lebanese Shia militia now deeply engaged in Syria fighting for the government.
It comes after an escalation of tit-for-tat attacks on Shia and Sunni leaders and communities in Beirut and the northern city of Tripoli that threatens to trigger all-out fighting between Shia and Sunni Muslims.
The precarious religious balance in Lebanon could be upset by further attacks on Hezbollah. Lebanon's Shia Amal opposition has long depended on Hezbollah to protect it, but it now fears that the larger community of Sunni Muslims will see it as an Iranian fifth column seeking to undermine Lebanon's sovereignty. Whereas the 1975-90 civil war largely pitted Lebanon's Christian minority against the Muslim majority, the Christians now fear that they could be caught in the crossfire between al-Qa'ida-linked jihadists and Iranian-backed Shia militias.
Assassinations, gunfights and car bombs have claimed hundreds of lives, culminating in two blasts outside mosques in Tripoli in August that killed 40 people and wounded 400.
Iran lamely tried to blame Israel for Monday night's atrocity in an attempt to dampen tensions, but Tehran's proxies in Lebanon are sure to respond with revenge attacks on prominent Sunni politicians. The spectre terrifying the Muslim world is that of outright war between Shia and Sunni Islam. The escalating hostility between Saudi Arabia and Iran could quickly spark new flare-ups across the Middle East. Already tensions in Bahrain, Libya and northern Yemen have divided the Muslim population along sectarian lines.
Saudi Arabia, fearful of what it sees as encirclement by Iranian-backed Shia rivals, is now fearful that a nuclear deal between Iran and the West will remove all restraint on Tehran and encourage Iran and Shia militants to threaten Saudi Arabia's internal stability and privileged religious position in the Muslim world. The bombs in Beirut might be the catalysts for an ideological struggle that could devastate global Islam.