Fighters from the Shiite guerrilla group helped Syrian forces batter the rebel-held town of Qusair for three weeks until they finally overran it this week in a significant victory for Assad's regime. Many Sunni hard-liners have taken Hezbollah's intervention as a declaration of war by Shiites against Sunnis.
That could have dangerous implications not only for Syria's conflict but for the entire region.
Calls for jihad by Sunni clerics could increase the flow of foreign militants into Syria to fight alongside the rebels. Sunni Arab powers like Saudi Arabia and other Gulf countries, who see the war as a way to break the influence of Assad's Shiite ally Iran, may step up weapons supplies to Syria's rebels to counterbalance Hezbollah.
Already several thousand foreign militants from across the Arab world and as far away as Chechnya and Somalia are believed to be fighting among the rebels. Some have close ties to al-Qaida. Their presence has been a major reason the United States is reluctant to help arm the rebel movement.
It could also fuel the fires of conflict in Syria's neighbors. Hezbollah's intervention in Syria threatens to bring that country's conflict even further into Lebanon, where rebels have vowed to retaliate with attacks on the Shiite group's home turf. It has also enraged Sunnis in Lebanon, who resent Hezbollah's political domination in their country and the weak government's inability to rein them in.
Speaking in the northern Lebanese city of Tripoli on Wednesday, a founder of the hard-line Sunni Salafi movement in Lebanon, Sheik Islam al-Shahal, said it was time for Sunnis to fight back against what he called Shiite Iran's control of Lebanon through Hezbollah.
"The (Iranian) occupation of Lebanon must be confronted by preparing every Sunni family and every young Sunni man to defend his faith, his home and his honor. We are clearly targeted," he said.
Iraq has also seen a dangerous upsurge in tit-for-tat attacks and bombings between Sunnis and Shiites in recent weeks, raising fears of a revival of the sectarian slaughter of 2005-2008. Syria's conflict is intertwined with Iraq: al-Qaida's branch in Iraq is connected with jihadi fighters in Syria and the two are believed to trade resources. The heightened sectarian tone across the region is likely to fuel determination among Iraq's Sunni minority to stand up against the country's Shiite leadership.
On Friday, the acting head of the main Western-backed opposition group, the Syrian National Council, cited the possibility of a sectarian war as a reason for the West to intervene.
"The problem will be widespread all over the Middle East," George Sabra told The Associated Press in Copenhagen, Denmark. "The international community should act now. Otherwise it will be a real dangerous conflict between Sunnis and Shias."
The trajectory of Syria's civil war also highlights the two tracks of the Arab Spring. It began in late 2010 and early 2011 with a wave of uprisings calling for greater democracy, openness and people power that led to the toppling of autocrats in Tunisia, Egypt, Libya and Yemen. But parallel to the desire for democracy has been a strong rise in religious identity politics.
Islamist ultraconservatives have risen in power and influence in multiple countries, espousing a sort of Sunni triumphalism as part of their dream of seeing rule by Shariah, or Islamic law.
The ultraconservative movement known as Salafis along with its more extremist jihadi wing has been the most assertive in fueling the sectarian fires, through mosques and the blossoming number of Salafi TV stations. In Egypt, they also have a powerful political platform following election victories that made them the second-strongest force in parliament after the Muslim Brotherhood.
The resonance of identity politics is so strong that even Egypt, which has a minuscule Shiite population, saw a panic over Shiism this year. When President Mohammed Morsi of the Brotherhood allowed direct flights of tourists from Iran for the first time in decades as part of an attempt to improve ties with Tehran, Salafi politicians and clerics raised an uproar, warning of Shiites spreading their creed. The flights were stopped, but Salafis continue to preach against them.
Syria's conflict began like others of the Arab Spring, as an uprising against Assad's authoritarian rule. As the bloodshed dragged on, however, sectarian hatreds have grown steadily. Alawites, an offshoot sect of Shiism, make up the backbone of Assad's regime, and Assad himself is an Alawite, while the rebellion has overwhelmingly drawn from Syria's Sunni Muslim majority. With each massacre and atrocity, each community in Syria turns more to its own and views the war as "us vs. them."
On a geopolitical level, Syria's war has morphed into a proxy fight. Shiite Iran has strongly backed Assad, while Sunni Arab nations have backed the rebels in hopes that Assad's fall would break the alliance that gives Iran their chief regional rival influence in the Arab world.
Now those using the region's heightened sectarian tone are casting it as an outright religious fight between the two main sects of Islam, which split in the 7th century in a dispute over who should lead Muslims after the death of the Prophet Muhammad.
Hezbollah has depicted its entry into the Syria fight in sectarian terms, saying it must defend against "takfiris" a term for Sunni radicals. Its fighters killed in the Qusair siege were touted as martyrs for Sayida Zeinab, a shrine near Damascus revered by Shiites.
Sunni hard-liners have long supported the Syrian rebels and frequently vented against Shiites and Alawites, whom the Salafis consider infidels. But even they usually tried to depict the fight as a revolutionary movement, not sectarian. Now even some moderates are turning toward the sectarian view.
A week ago, the influential Egyptian cleric Yousef al-Qaradawi angrily denounced Hezbollah Arabic for "party of God" as "the party of Satan" and called on Sunnis to join the jihad in Syria. A popular television preacher linked to the Brotherhood, Al-Qaradawi had in the past called for better ties between Sunnis and Shiites and praised Hezbollah in its fights against Israel.
In his sermon, he angrily said Iran wants "continued massacres to kill Sunnis."
"How could 100 million Shiites defeat 1.7 billion (Sunnis)?" he asked, using worldwide population figures for the two communities. "Only because (Sunni) Muslims are weak."
Saudi Arabia's top cleric, Sheik Abdulaziz bin Al Sheikh, on Thursday issued the strongest condemnation yet of Hezbollah's role in Syria, urging politicians and Muslim scholars to take "effective steps to deter its aggression" on Syria.
At a meeting a day earlier, foreign ministers from Gulf nations called Hezbollah a "terrorist organization" and vowed steps against the group's assets on their soil. They warned that Hezbollah was trying "to change the balance of the region and drag it into the furnace of the Syrian crisis."
On Salafi TV stations and in mosque sermons, meanwhile, clerics have stepped up their denunciations of Shiites and their calls for Sunnis to act.
"The Sunnis have to wake up from their coma," Egyptian Sheik Osama Suleiman said on a Salafi talk show last month. "This is what the Shiites are, this is their hidden wickedness. This is their belief: If you kill a Sunni you enter heaven. That's their belief."
But not all hard-liners see the conflict as a fight against Shiites.
Syria's revolution is "to bring down the regime ... If it becomes sectarian, then the revolution will be lost," said Mohammed Abu Samra, head of the Islamist Party the political arm of Egypt's Islamic Jihad, a former militant group that renounced violence and entered politics.
He worries that the rhetoric will affect younger jihadis.
"They became jihadis out of fervor and don't necessarily have the right consciousness and study," he said. "They might understand things wrong and go to fight jihad in Syria because they see it as a sectarian war."