Iranian troops are fighting in neighboring Syria to support the dictatorship there. The civil war in Syria is in its third year and it's become, in large part, a fight between the two main branches of Islam -- the Sunni and the Shi'a -- a rift that extends through the region.
There was panic and chaos in the first moments after Tuesday's attacks.
In the morning, a suicide bomber on a motorcycle detonated himself just outside Iran's embassy gate. Moments later, a much larger car bomb exploded down the street. Most of those killed were innocent bystanders.
A little known extremist group with ties to al Qaeda -- the Abdullah Azzam Brigades -- claimed responsibility for the bombings.
The group said the attack was in retaliation for Iran's support of the Assad regime in neighboring Syria.
Iran is the main backer of the Lebanese militia Hezbollah, which has sent thousands of fighters to join Assad's forces. In the past few months, those fighters have tipped the balance in favor of the Syrian government troops.
This is not the first time that Syria's war has spilled over into neighboring countries. But the attack on the Iranian embassy has raised fears the violence could escalate.
Essentially this boils down to a struggle for regional power between the two main branches of Islam. Iran and Hezbollah are Shiite, which makes them natural allies to the Assad regime in Syria, which is fighting a Sunni rebellion that is being funded by oil-rich Sunni countries like Qatar and Saudi Arabia.
As for Lebanon, it is in a very precarious situation because it has a large Sunni population and a large Shiite population. The country fought its own brutal civil war for 15 long years that ended in 1990. Nobody in Lebanon wants to see that happen again.