s the conflict in Syria spills across the border into Lebanon, sectarian violence there has risen to levels unseen in recent years. A car bomb exploded in Beirut's Hezbollah-controlled suburbs on Jan. 2, killing at least five people and wounding dozens more. It was just the latest attack in a shadow war between Sunni and Shiite factions that now risks spiraling out of control: Less than a week earlier, a top advisor to an anti-Assad former prime minister was killed in downtown Beirut; in November, a double suicide bombing targeted the Iranian embassy in Lebanon.

The most recent car bombing, as well as the attack on the Iranian Embassy, appears to be the work of Sunni extremist groups, which are rapidly gaining strength across Lebanon. In a microcosm of the Syrian civil war, Lebanese Sunni militants now regularly engage in violent clashes with pro-Assad Shiite and Alawite rivals in hotspots such as the cities of Saida and Tripoli, and the eastern Bekaa Valley.

With the death toll climbing, fears are mounting that the fighting could push the increasingly embattled Sunni community toward large-scale militia building. The Lebanese Army recently arrested the Saudi leader of the al Qaeda affiliate in Lebanon, which claimed responsibility for the Iranian Embassy attack -- but homegrown Sunni militants may be quick to step up and take his place. Sunni groups have already begun to organize in order to funnel aid to Syrian rebels and combat Hezbollah, and some have even begun to clash with the Lebanese Army, once considered the sole unifying institution in the country but now increasingly seen as a pawn in sectarian politics.

Historically, the Sunni community in Lebanon has had difficulty mobilizing militarily. Although the country's other sects have produced militias to protect their communities' interests, the Sunnis have been hampered by the absence of a dominant party capable of unifying and rallying their sect. Instead, Sunni militancy in Lebanon has traditionally been driven by groups with roots in the refugee Palestinian communities and the international Salafi-jihadi movement. These groups, which have carried out most of the attacks against Hezbollah and Assad supporters so far, have failed to garner significant popular support.

However, the political, social, and military dynamics affecting mainstream Lebanese Sunnis suggests that their calculus for large-scale militia building may be changing. The past decade has seen dramatic changes in the Sunni community's standing vis-à-vis other confessional groups. Perhaps the most important has been the failure of the Sunni political agenda following the 2005 assassination of former Prime Minister Rafiq Hariri. The Sunni-dominated government that was elected after his death was determined to root out the vestiges of Syrian influence and end the militarization of Hezbollah -- but it was ultimately frustrated. The government's disastrous attempt to dismantle a Hezbollah-operated telecommunications system in May 2008 led the Shiite party to invade predominantly Sunni neighborhoods in Beirut, leaving the Sunni community fragmented and dispirited.

The inability of Sunni politicians to protect their community's interests has created space for fundamentalist leaders to claim the role of community defenders. In recent years, numerous Islamist groups -- which often receive funding from sympathetic networks in the Gulf -- have gathered followers by speaking to the concerns of the Sunni street. And while there is growing support for Syrian rebels among the mainstream Sunni community and its political elites, it has been the Salafist militants that have taken the lead in actual fighting in Syria.

The first Sunni rebel group fighting in Syria under a Lebanese commander -- and probably still the best known -- is Jund al-Sham ("Soldiers of the Levant"). It operates in Homs under Khaled Mahmoud, a well-known militant who fights under the nom de guerre Abu Suleiman al-Muhajer. Another Salafi group, led until recently by firebrand Lebanese cleric Ahmad al-Assir, is Kataib al-Muqawama al-Hurr ("The Free Resistance Brigades"). It was formed in April 2013 as a volunteer force of Lebanese Sunnis committed to opposing Hezbollah, but was weakened after clashing with the Lebanese Army in June 2013. Highlighting the growing cross-border cooperation between extremists, Jabhat al-Nusra, the al Qaeda-affiliated terrorist group in Syria, has reportedly had a presence in Lebanon since at least December 2012.

The epicenter for Sunni militancy in Lebanon appears to be the northern city of Tripoli. The city is the birthplace of the country's Salafi movement and many young men from Sunni neighborhoods have allegedly joined Jabhat al-Nusra, according to Lebanese security officials. Tripoli is also a one of the Lebanon's most volatile sectarian fault lines, with Sunnis regularly clashing with the city's small Alawite community. Over the last two years, the violence has claimed hundreds of lives.

The perfect storm now appears to be brewing in Lebanon. Hariri's assassination and the subsequent failure of Sunni elites to fill the leadership vacuum, the rise of Hezbollah as a military and political powerhouse, and Assad's crackdown on the predominantly Sunni opposition has fundamentally changed the dynamics of Sunni political participation and activism. The Syrian war has provided new and dangerous opportunities for militancy: Lebanese Sunnis are now learning to organize militarily, building bridges with foreign militant groups, and gaining tactical experience on the battlefield. Nearly a million refugees from Syria, who are predominantly Sunnis, have drastically altered Lebanon's confessional demographics.

Thus far, the fragmented nature of the Sunni community and the reluctance of all parties to become ensnared in an open a military confrontation has prevented civil war. However, as sectarian tensions grow, the Sunni community may come to perceive the current conflict as an existential threat. In this case, leaders -- moderates and extremists alike -- willing to employ violence to protect their community's interests may gain power. Such a shift could pose a significant threat to the confessional balance of power in Lebanon and, ultimately, could incite a deadly new civil war.