For Hezbollah, it may well be an existential battle. If Assad falls, Hezbollah's supply line of Iranian weapons through Syrian territory would dry up and it could become increasingly isolated in the region.
At the same time, Hezbollah, a Shiite Muslim group, is raising the sectarian stakes in Lebanon by declaring war on Syria's rebels, most of them Sunni Muslims.
Lebanon and Syria share the same uneasy mix of Sunnis, Shiites, Christians and Alawites, or followers of an offshoot of Shiite Islam. In trying to defeat the rebels, Assad relies on support from minority Shiites, Christians and his fellow Alawites.
On Beirut's beach promenade, opinions about Hezbollah's new strategy seemed to fall along religious lines.
Mahmoud Masoud, a Sunni, said he fears Lebanon will become more unstable. "I don't want to see everything I've worked for and my country fall apart of because of a certain group's interests," he said of Hezbollah.
Tamam Alameh, a Shiite, sided with Hezbollah. "The Syrians helped Lebanon a lot. We should help them and rid them of the conflict in their country," he said.
The rockets struck early Sunday in south Beirut, an unusual type of attack. In occasional sectarian flare-ups since the end of Lebanon's 15-year civil war in 1990, rival groups have mostly fought in the streets.
One rocket hit a car dealership in the Mar Mikhael district, wounding four Syrian workers, badly damaging two cars, and spraying others with shrapnel. Part of the rocket's main body was embedded in the ground, where a Lebanese soldier measured its diameter.
The second rocket tore through a second-floor apartment in the Chiyah district, about two kilometers (one mile) away. It damaged a living room, but no one was hurt.
Rocket launchers were later found in the woods in a predominantly Christian and Druse area southeast of Beirut, security officials said.
There was no claim of responsibility, but the attack was widely portrayed as retaliation for Nasrallah's defiant speech and Hezbollah's participation in a regime offensive in the past week on the rebel-held Syrian town of Qusair, near Lebanon. The regime has pushed back the rebels in Qusair, but has so far failed to dislodge them.
In an amateur video posted online a few days ago, a rebel commander threatened to hit Hezbollah targets in south Beirut in retaliation for the militia's part in the fight for Qusair.
Some said the rockets are just one sign that Lebanon is becoming a battleground.
"Nasrallah declared that he is part of the Syrian civil war," said Nadim Koteich, a TV talk show host and frequent Hezbollah critic. "He did not tell the Lebanese people why he thinks this civil war will not come to Lebanon."
In the northern Lebanese city of Tripoli, Sunni opponents and Alawite supporters of the Assad regime have repeatedly fought with mortar shells, machine guns and grenades since the start of the Syria conflict.
The latest round in the past week, apparently sparked by the Qusair offensive, was the longest and deadliest so far, with more than two dozen killed and more than 200 hurt.
Lebanese Sunnis have also entered the Syria battle, joining rebel units, though in a less-organized way than Hezbollah.
Hezbollah remains the most powerful group in Lebanon, backed by a military wing armed with tens of thousands of Iranian missiles.
Despite the risk of a backlash over the involvement in Syria, Hezbollah appears to be banking on continued support from Lebanon's Shiites, for whom it provides an extensive social support system.
Sheikh Nabil Kaouk, Hezbollah's commander in south Lebanon, signaled a tough line Sunday. "If the rockets were meant to terrorize us and pressure us into changing our position (on Syria), they have failed to do that," he told a Hezbollah function.
The Arab world's Sunni leaders were predictably harsh on Nasrallah.
In Bahrain, Foreign Minister Sheik Khalid bin Ahmed Al Khalifa described the Hezbollah chief as a "terrorist" and said it was Lebanon's "national and religious duty" to remove him from his influential position, according to the official Bahrain News Agency.
In Cairo, Arab League Secretary General Nabil Elaraby condemned Sunday's rocket attack but also urged Hezbollah to stop interfering in the Syrian civil war.
It is not known how many men Hezbollah has sent to Syria, but the militia's trained fighters fill a dire need for Assad's army.
Regime troops have been stretched thin, both because of defections at the start of the conflict and because only the most politically loyal have been sent into battle.
It is unclear how Hezbollah's new strategy will play out, said Peter Harling of the International Crisis Group think tank.
"They do see this as something that can redefine the rules of the game region-wide, and they are mustering all the strength they have to win this," he said of Hezbollah. "But it is doubtful strength alone can achieve this, as the regime itself has shown."
The Assad government, meanwhile, confirmed Sunday that it has agreed in principle to attend U.N.-sponsored talks with opposition representatives in Geneva next month on ending the civil war.
Syrian Foreign Minister Walid al-Moallem said during a visit to Iraq that such talks present a "good opportunity for a political solution for the crisis in Syria." He did not say under what terms Assad would dispatch representatives.
The date, agenda and list of participants for the conference remain unclear, and wide gaps persist about its objectives.
Syrian opposition leaders have said they are willing to attend the Geneva talks, but that Assad's departure from power must top the agenda. Assad said this month that his future won't be determined by international talks and that he will only step down after elections are held.
Al-Moallem's statement puts more pressure on Syria's fractured political opposition to signal acceptance as well. The main bloc, the Syrian National Coalition, met in Istanbul for a fourth day Sunday to come up with a unified position on the proposed peace talks, elect new leaders and expand membership.
Louay Safi, a senior member of the coalition, said participants were bogged down in talks about the expansion and won't be able to issue a statement on the Geneva talks until membership issues are settled.
Associated Press writers Zeina Karam and Yasmine Saker in Beirut, Brian Murphy in Dubai and Aya Batrawy in Cairo contributed reporting.