In recent weeks, Hezbollah's heavily guarded stronghold in south Beirut has been bombed repeatedly, with dozens of people killed and hundreds injured, in apparent retaliation for the Shiite movement fighting alongside Syrian President Bashar Assad's force to preserve his Damascus regime against rebel groups drawn from Syria's Sunni majority.
These attacks "point to two disturbing trends: the growing radicalization of under-privileged Sunni Muslims and the entry of al-Qaida's two Syrian factions into Lebanon," Mideast analyst Alexander Corbeil observed in a report by the Beirut Center of the Carnegie Endowment for International Peace.
Syria's rebellion has become dominated by jihadists, many of them from neighboring Iraq, with thousands of foreign militants from across the world.
Hezbollah is aiding Assad's dictatorship because Damascus had helped the organization since revolutionary Iran set it up in 1982 during Israel's invasion of Lebanon.
Damascus has used the group as a proxy to fight Israel. But Hezbollah needs the Assad regime to remain in power and maintain the flow of Iranian arms and supplies.
As the Syrian war has inevitably spilled into Lebanon, destabilizing the volatile Mediterranean nation with more than 1 million refugees, mostly Sunnis, and straining its already fragile economy to breaking point, Sunni anger has gone toxic.
With Sunni political leaders pushed from power by a Hezbollah-led alliance backed by the party's immense firepower, which far outguns the poorly equipped army, the country has been without a government since March.
Hezbollah has blocked any proposed government it won't be able to dominate.
The Dec. 27 car bomb assassination of key Sunni figure Mohammed Chatah, a highly respected moderate and key adviser to Saad Hariri, leader of the Future Movement, the Sunnis' main party, effectively crushed the Sunni center.
The lack of moderate leadership is illustrated most starkly by the absence of Hariri himself. He fled the country, fearing assassination, after Hezbollah engineered the collapse of his administration two years ago.
He hops between Paris and Riyadh, capital of Saudi Arabia, his patron and protector.
Chatah's murder shows Hariri's fears are not far-fetched. On top of that, his father, Rafik Hariri, Lebanon's only real statesmen and a towering figure who rebuilt the country from the ruins of its 1975-90 civil war, was assassinated in a suicide bombing in Beirut Feb. 14, 2005, as he sought to end Syrian domination.
The Syrian regime was widely seen as being behind that killing, and those of a dozen other anti-Syrian figures who followed. Damascus denied that, but a U.N.-mandated tribunal has indicted five Hezbollah members for killing Hariri.
The trial of the suspects -- who Hezbollah refuses to hand over -- is scheduled to start in The Hague Thursday. It could trigger another bout of sectarian savagery.
In the political vacuum, with the economy crumbling, Lebanon's Sunnis are turning increasingly to the sect's hardliners who have been building up their forces, largely in Palestinian refugee camps.
"This inability to form a government acceptable to all parties, a common theme in Beirut, has a direct impact on the security situation," Corbeil observed.
"Without the guidance of a cross-sectarian executive branch, the country's main guarantor of security, the Lebanese Armed Forces, cannot effectively police Sunni extremism and other violent symptoms of the Syrian civil war."
Saudi Arabia, desperate to prevent its Lebanese outpost falling into Iran's orbit, has pledged $3 billion to bolster Lebanon's military.
That could take two- to three years, so it will have little impact on the current crisis.
But it may have spurred Iran, which doesn't want to jeopardize a possible rapprochement with the United States, to try to calm things down in Lebanon.
Zarif didn't give much away when he left Monday. But there's speculation he urged Nasrallah to go along with a new government, if only to dampen fears of a major sectarian eruption in Lebanon that could upset a possible deal with Washington.