When two Islamic State suicide bombers walked into the bustling south Beirut suburbs Thursday and detonated their vests, killing at least 43 people, it was the deadliest blast to hit the Lebanese capital for a quarter century.
Because of the target – a largely Shia neighbourhood where many residents are supporters of the Hezbollah militia – it was quickly seen as spillover from the brutal war next door in Syria, where Hezbollah has sent fighters to support President Bashar al-Assad's regime. In the immediate aftermath of Thursday's attack, Hezbollah issued a statement declaring itself enmeshed in a "long war" against the so-called Islamic State, the most extreme of an array of Sunni Muslim groups fighting Mr. al-Assad's regime, which is dominated by followers of an offshoot of Shia Islam.
What's not yet clear is the extent to which the Islamic State intends to take the long war into Lebanon, a country that has been struggling since the 2011 outbreak of Syria's conflict to keep the flames from sweeping across the border. That Lebanon has thus far remained largely at peace is something of a miracle, one many attribute to memories Lebanese have of their country's own 1975 to 1990 civil war, which claimed 120,000 lives.
But the kindling is here for another inferno. Lebanon has its own deep Sunni-Shia divide, and fighters on both sides – including Hezbollah as well as some volunteers who have joined their fellow Sunnis in the battle to topple Mr. Assad – that are already clashing on the other side of the border.
"We have escaped war for the four-and-a-half years, but I'm not sure we can continue escaping it," Sarkis Naoum, a columnist at the respected an-Nahar newspaper, said in an interview conducted just as the blasts were going off in south Beirut on Thursday. "The Sunnis and the Shia are at war, just not yet in Lebanon."
Lebanese, Mr. Naoum said, know another civil war "will destroy the country." But that didn't mean the country couldn't yet go down that road. Lebanon has been teetering on the edge of disaster since the 2005 assassination of Rafiq al-Hariri, a popular former prime minister and prominent Sunni Muslim. Many Lebanese blame the murder on Mr. Assad, whose army was then occupying Lebanon, and Hezbollah.
The arrival of an estimated 1.5 million to two million Syrian refugees – most of them Sunnis – since 2011 adds to the tensions in this country that had a prewar population of just 4.4 million and a delicate sectarian balance.
Among the suspects in the bombings were two Palestinians and a Syrian (a third would-be suicide bomber was killed by an earlier blast before he could detonate his own device). Police believed the attackers emerged from the Burj al-Barajneh refugee camp, where thousands of refugees from Syria's civil war now crowd in with generations of Palestinian refugees who fled here following the 1948 and 1967 Arab-Israeli wars.
Like Syria, Lebanon is an arena where Saudia Arabia and Iran – the capitals of the Sunni and Shia worlds – are furiously jousting for influence. The presidential palace in Beirut has sat empty for the past 18 months as Sunni and Shia-dominated political blocs have been unwilling to compromise with the other side on a candidate. (Under the country's power-sharing constitution, the president, who is chosen by parliament, must be a Maronite Christian, though Christians are a dwindling and divided political force in the country.)
The political gridlock reached such a state that garbage in Beirut and other parts of the country went uncollected for much of the summer, prompting anti-government protests – under the unifying banner of #YouStink – that were encouragingly attended by people from all religious backgrounds.
Much of the garbage has since been moved out of at least the centre of Beirut to informal dumps around the country, but the parliament still struggled this week to pass even basic legislation required to let the country receive millions of dollars in desperately needed World Bank aid.
"Lebanon is caught in the nexus of the Sunni and Shia conflict, between Iran and Saudi Arabia, and it's in the middle of the Syrian refugee crisis," said Nadim Houry, director of the Beirut office of Human Rights Watch. "They're playing chicken, all of the parties, thinking the other guy is going to blink first."
So far neither patron has ordered their proxies to bring the fighting into Lebanon, but there's a fear that could change if a rogue actor – such as Islamic State – decided to open a front in the country. There have already been a series of gun battles in the border areas, alternately pitting the Lebanese army or Hezbollah against the Islamic State or other Syrian rebel groups.
Friday began nervously in Shatila, another of Beirut's troubled refugee camps where those fleeing Syria's war have moved in beside generations of Palestinians. Worries that Thursday's suicide bombings would provoke more violence gradually subsided over the course of the day as a feared Shia backlash never materialized.
Hanan Laham, a 52-year-old Syrian-Palestinian mother of five, said residents of the camp were themselves trying to keep the Islamic State from taking root there, knowing the group's presence would bring trouble with the Lebanese army and Hezbollah.
"If our leaders had not been calm and level-headed yesterday," she said, "it could have started a whole crisis."