Supporters of Hezbollah leader Sayyed Hassan Nasrallah wave flags and a picture depicting Nasrallah, Syria's late President Hafez al-Assad, and Syrian President Bashar al-Assad, 2012. (Ali Hashisho / Courtesy Reuters)

Hassan Nasrallah, the leader of Hezbollah, appeared to cross a Rubicon last week. In a defiant speech on May 25, he emphasized the Lebanese Shia militant group’s unbridled support for the Assad regime in Syria. In doing so, he lifted the veil of secrecy that had surrounded Hezbollah’s deepening involvement in Syria. Of course, Hezbollah’s backing of Syria had never been in question. Yet the organization had worked assiduously to cover its tracks, even as the number of funerals for those “martyred” in Syria mounted.

Beyond publicly confirming what everyone already assumed, the speech, which was staunchly sectarian, signals a critical turning point for Hezbollah. It could mark the group’s transformation from resistance movement to sectarian militia. Nasrallah delivered an unprecedented and stinging criticism of Sunni hard-liners in Syria. He heralded a “new stage” in Lebanon’s struggle against external threats, adding a jihadist-controlled Syria to a list of enemies that already includes Israel and the United States.

Hezbollah’s ongoing metamorphosis will provoke a strong response from Syrian rebels as well as from Lebanon’s increasingly radicalized Sunni community. In the wake of the speech, Syrian rebels increasingly threaten reprisals against Hezbollah, which they often refer to as the hizb ash-shaytan, which means party of the devil. General Salim Idriss, chief of staff of the rebel Supreme Military Council has warned that he will not be able to "restrain the fighters" who will take "all measures" if Hezbollah does not withdraw from Lebanon. "We will chase Hezbollah to hell," Idriss continued. Colonel Abdul-Jabbar al-Aqidi, commander the Military Council in Aleppo, threatened "to strike at your stronghold in Dahiyeh,” which are Beirut's southern suburbs.

Radicalized elements in Lebanon's Sunni community appear to have launched rocket attacks at Hezbollah’s Beirut stronghold and fired on a Shia shrine outside the Bekaa valley city of Baalbek, another Hezbollah stronghold. Even moderate Sunni organizations such as the Muslim Scholars Association of Lebanon are calling on the Sunni community to support the Syrian rebels in every way, including fighting.

All that spells more uncertainty for an already unstable Lebanon. And more broadly, Hezbollah’s “all in” decision in Syria and its sectarian pivot underscore the powerful sectarian dynamic building across the Arab world. On the first of June, Yusuf al-Qaradawi, the prominent Sunni Sheik, issued a fatwa deeming Hezbollah and Iran to be infidels and calling on Sunnis worldwide to join the battle in Syria.

At the outset of the Syrian uprising, Hezbollah had adopted a nuanced stance. Nasrallah and other Hezbollah leaders underscored the importance of dialogue and reform in Syria and reached out quietly to various opposition groups. (In his recent address, Nasrallah even lamented his failed attempts to promote dialogue in Syria, blaming the rebels.)

But as the conflict deepened and armed rebels increasingly threatened the Syrian regime, Hezbollah and Iran solidified their support for Assad. Hezbollah initially provided discreet advice and training to Syrian forces, particularly the Alawite paramilitary groups known as the shabiha, which are believed to be responsible for undertaking Sunni massacres and other sectarian violence. Over time, though, Hezbollah headed to the front lines. Today, the number of Hezbollah fighters inside Syria is estimated to be in the thousands. It fights alongside the regime to regain control of Qusayr, a strategic town southwest of Homs, and it has even deployed a security force around the revered Sayida Zeinab Shiite shrine in Damascus.

It is the battle for Qusayr that appears to have been the catalyst for Hezbollah’s open embrace of the Syrian regime and its tirade against Sunni extremists. Qusayr is of paramount importance to the Assad regime, Hezbollah, and the Syrian rebels. For Assad, its capture would help secure the strategic corridor leading from Damascus to the Alawite-dominated area along Syria’s Mediterranean coast. For both Hezbollah and the rebels, the city provides a valuable passage for arms between Lebanon and Syria (and vice versa).

The stakes in Qusayr are high, and the Syrian regime -- and now Hezbollah -- has pulled out all the stops to win it. As a result, Hezbollah has suffered significant casualties. Reportedly the group lost dozens of fighters (some estimates are as high as 75), the highest death toll in any battle since Hezbollah’s 2006 war with Israel. It is therefore understandable that Nasrallah no longer felt able to deny Hezbollah’s intense involvement in Syria. Yet it is ironic that he would choose the commemoration of Israel’s 2000 withdrawal from Lebanon, which was the ultimate triumph of Hezbollah’s resistance efforts, to come clean and introduce a new, sectarian narrative for his group. It was almost breathtaking to hear Nasrallah contort his group’s traditional talk about resistance against Israel -- a broad appeal to the Arab street -- into a thinly veiled defense of Shia interests. Hezbollah’s survival is now grounded firmly in its alliance with two brutish regimes in the Shia orbit. The galvanizing narrative of resistance to Israel is fast being overtaken by an even more powerful, yet divisive, sectarian dynamic.

Ever the political strategist, Nasrallah attempted to spin his arguments to avoid charges of sectarianism. Even as he harped on the threat posed by takfiris in Syria, he noted that they threaten all Lebanese: Christian, Druze, Shia, and Sunni. He referenced other cases of takfiri extremism, noting in particular their ideological opposition to holding elections and their grisly tactics. In Iraq, he reminded his audience, more Sunnis than Shias or Christians were killed by al Qaeda jihadists. Finally, he took the opportunity to acknowledge that the Lebanese are divided on Syria, but exhorted both sides to keep the fight within Syria’s borders, not bring it to Lebanon.

Despite Nasrallah’s impassioned delivery, his words rang hollow. Indeed, as if to make the point, two rockets were fired on the Hezbollah stronghold in Beirut’s southern suburbs just hours after the speech. Although the rockets caused relatively minimal damage, they highlighted the disconnect between Nasrallah’s vision and reality.

Nasrallah might hope that Hezbollah’s unalloyed support for the Assad regime in Syria’s increasingly brutal sectarian civil war will have no blowback in Lebanon. He may seek to cloak Hezbollah’s turn toward sectarianism by making superficial appeals to broader swaths of Lebanon’s diverse population. But it is hard to see how he will end up being anything other than disappointed.

In Lebanon, Hezbollah’s blatant support for the Assad regime will likely spur more serious provocations, particularly from an increasingly enraged and emboldened Sunni community. Although Lebanon will not descend into all-out civil war -- all of the key players there seek to avoid a repeat of the past -- it may well enter a more dangerous period of instability. Meanwhile, Hezbollah’s credibility in the Arab and Muslim world will likely be damaged irreparably, which will place the group increasingly at odds with the Sunni community and diminish its appeal more broadly.

The positive wave unleashed by the Arab rebellions has given way to a more dangerous undercurrent. Fueled by the growing influence of identity politics, tribalism, and sectarian dynamics, the stakes for many are existential. Hezbollah’s embrace of a sectarian agenda is emblematic of the deeper dynamic that is reshaping the region, particularly the Levant. Although it may be motivated by self-preservation and a deeply ingrained survival instinct, Hezbollah’s turn toward sectarianism promises to unleash far more serious challenges, imperiling Hezbollah and likely marking the end of Lebanon’s relative stability.