(The Christian Science Monitor) Hezbollah's military support for Syria's regime, which is fighting a mainly Sunni Muslim armed opposition, has divided Lebanon. But since June 2014, when a year-long bombing campaign ended, the war next door hasn't afflicted the Shi'ite militant group's strongholds in Lebanon. 

That changed Thursday evening, when a double suicide bombing left at least 43 people dead and more than 230 wounded. The attack, claimed by the Islamic State, targeted a district long dominated by Hezbollah.

But for many in Beirut's mainly Shi'ite southern suburbs, the suicide bombings were not unexpected. Rather, the surprise was why there was such a long gap since the last attack 17 months ago. And fears are running high of a fresh wave of sectarian attacks on their communities. 

“We ask ourselves all the time when the next attack will come,” said Khodr Slim, a young Shi'ite who was among the crowd gathered at the scene of the bombing. “We live with this concern every day.” 

The blasts occurred minutes apart outside a Shi'ite meeting hall, tucked into the narrow streets of Bourj al-Barajneh. Hezbollah gunmen kept the crowd back by firing shots from rifles while others directed ambulances to the scene through the dense throng of onlookers.

IS released a statement claiming responsibility: “Soldiers of the caliphate blew themselves up in the stronghold of the heretics, and after the apostates crowded around the site of the explosion a second martyr blew himself up using his explosive belt.” 

Top Lebanese officials met Friday morning to assess the security situation amid widespread denunciations from across the political divide.

“The barbaric crime in Bourj al-Barajneh didn’t target a certain area or sect, but smeared Lebanon with blood,” said Prime Minister Tammam Salam.

Previously, Hezbollah-supporting regions of the country were struck by bombings 14 times, including suicide attacks, between July 2013 and June 2014. The spate of attacks left nearly 100 people dead and almost 900 wounded.

Most of the car bombs were manufactured in workshops in Syria before being driven over the border into Lebanon. The majority of attacks were claimed by al-Qaeda’s Syrian affiliate. IS claimed responsibility for one of the bombings. Among the targets was the Iranian embassy in Beirut, hit by two suicide bombers in November 2013, killing 23 people.

That month, Hezbollah launched an offensive to seal off Lebanon’s northeastern border with Syria. By April 2014, the Syrian rebels had been ousted from the area, and the spate of suicide car bombings slowed.

At the same time, the Lebanese army, police, and security services manned checkpoints at the main entrances to Beirut’s southern suburbs in order to stop suicide car bombings. The last such attack in June 2014 killed a policeman who challenged the driver of the vehicle at a checkpoint. 

Part of the success of the effort to stop the car bombs may lie in the fact that Lebanon is a small country, only two thirds the size of Connecticut. Its close knit demographics leave little operational space for bombers to plot and execute car bomb attacks without someone finding out. And its sometimes rival security services have coordinated closely to track potential militants, making dozens of arrests.

Still, it's hard to stop individuals on foot from detonating vest bombs or firing concealed weapons in crowded areas. Sources close to Hezbollah say that the party’s security officers have fretted for years over the potential for such attacks.

Hezbollah militants use bomb-sniffing dogs to regularly sweep the streets of southern Beirut in the early hours of the morning. Security cameras also keep watch on the bustling streets. Occasionally, Hezbollah members wearing municipal police uniforms sit beside army and police checkpoints in southern Beirut, reading the registration plates of each passing car by microphone to a central office where the numbers are checked off against a list of suspect vehicles.

Thursday’s bombings demonstrate the relative ease with which individual suicide bombers can infiltrate even a closely watched area like southern Beirut to perpetrate attacks. 

Another security headache for Hezbollah is a huge influx of Syrian refugees to Lebanon, mostly Sunnis. During the 2013–2014 bomb attacks, Hezbollah conducted a census of all Syrians living in areas under their control, and it continues to update this list.

The latest bombings will likely harden attitudes of the southern suburbs’ residents against Syrians living among them. The streets of southern Beirut were emptier than usual Friday morning, with most Syrians staying at home.