[Published by the Jerusalem Post under the title "New opponents, new challenges in the Middle East"]
Nusra leader Abu Muhammad al-Julani (left) and Hezbollah leader Hassan Nasrallah don't exactly see eye to eye.
Hezbollah is under pressure as the consequences of its ongoing intervention in Syria have come back to bite the terrorist organization.
There are increasing indications that the sectarian war raging in Iraq and Syria is now moving irrevocably into Lebanon.
The Shi'ite group is currently seeking to shore up its legitimacy by reminding its constituents, and other Lebanese citizens, of the role that gains it the most domestic sympathy – resistance against Israel. It is likely the strike at Mount Dov last week was part of this effort.
It is also, in its propaganda, somewhat oddly trying to assert that Israel and the Sunni jihadis of the Nusra Front and Islamic State are allies.
All this activity comes as the Nusra Front is demonstrating its ability to hit at Hezbollah across the border with increasing impunity.
Attacks by Sunnis in Lebanon are not new, and similar incidents have taken place throughout the Syrian civil war.
The longstanding tension in the Tripoli area between the mainly Alawi, pro-regime inhabitants of the Jebel Mohsen neighborhood and the mainly Sunni, pro-rebel Bab al-Tabbaneh neighborhood is continuing.
The Nusra Front is demonstrating its ability to hit at Hezbollah across the border with increasing impunity.
Hezbollah, in cooperation with the Lebanese Armed Forces (LAF), managed to stem a bombing campaign by the Sunnis in the Shi'ites' heartland of southern Beirut in the middle of 2013.
And tensions between Hezbollah supporters and the local Salafi leader Sheikh Ahmed al-Assir in June 2013 ended in pitched battles and the destruction of Assir's local power.
The current tension, however, differs from previous episodes.
It does not involve Hezbollah fighting much weaker local Sunni forces. This time, the movement is clashing directly with Syrian Sunnis.
The current phase began with the seizure of the Lebanese border town of Arsal in August by members of both the Nusra Front and Islamic State. They left with a number of captured Lebanese soldiers, some of whom have since been executed.
The LAF then tried to crack down on local support for the jihadis in Arsal, carrying out a large raid on the town in September, arresting hundreds accused of being Nusra Front members or for supporting the movement.
More importantly, most of the individuals in the crackdown were not Lebanese Sunnis but rather members of the 1.5 million Syrian Sunni refugees, now in Lebanon.
The Nusra Front then struck back hard in an operation whose stated goal was to "avenge Syrian refugees whose tents were burned" during the crackdown on Arsal.
Hundreds of fighters of the organization attacked from across the Syrian border, forming a line from Baalbek up to Arsal itself.
The attack wasn't directed against the LAF, but against Hezbollah's positions.
The attackers were eventually defeated (or the battle was intended to be a hit-and-run attack, depending on who one chooses to believe). But the jihadis fought a two-hour pitched battle with Hezbollah fighters near the village of Britel.
The Nusra Front overran a Hezbollah position, killing at least 11 of the movement's fighters.
The Sunnis filmed the attack, as well as its aftermath. The jihadis can be seen moving backwards through the Hezbollah position, removing equipment, nonchalantly ignoring the corpses of dead defenders.
The Britel battle represents an eruption into Lebanon of a wider campaign, in which Hezbollah and other pro-Assad forces have been desperately trying to clear out the Sunni jihadis from the Qalamun mountain range along the border and seal the line between Syria and Lebanon.
The Nusra Front and its allies are trying to establish a connecting route between Arsal and al-Zabadani, west of Damascus, long held by the rebels.
The fight for Qalamun has turned into a grinding affair for Hezbollah, costing the lives of many of its fighters, while it never quite seems to end. The Britel losses indicate the failure of the pro-Iranian bloc's efforts to finish this fight, and show that the direction of events, for now, at least, are in the Nusra Front's favor.
But the wider implications and challenges of the intensification of cross-border Sunni activity are political.
As its casualties in the seemingly unending Syrian war continue to mount, Hezbollah needs to redouble efforts to explain to its constituency why this sacrifice makes sense and how it fits into the movement's more familiar justifications for its existence.
Hence the increase in public statements by top officials, including leader Hassan Nasrallah.
Nasrallah paid a rare visit to Bekaa this week. In his speech, he sought to link the fight with the Sunni jihadis to Hezbollah's war with Israel.
"Victory will be the ally of the mujahideens in their fight against takfiri [apostate Muslims] and terrorist groups, the same way it was their ally in the confrontation against the Israeli enemy," Nasrallah said.
Interestingly, the Hezbollah leader didn't stress the military campaign in Bekaa, but rather boasted of the attack in the Mount Dov area, which he said showed "the resistance, which is always vigilant, will protect any attempt to attack Lebanon or its people."
Pro-Hezbollah publicists, meanwhile, are seeking to color in this picture with claims that Israel and the Nusra Front have reached an understanding with one another and are cooperating against Hezbollah, as Jean Aziz, a columnist at the pro-Hezbollah al-Akhbar wrote in a recent article.
These statements and claims notwithstanding, the main concern for Hezbollah and its supporters is the effect that the Nusra Front's offensive into Lebanon is having on the delicate balance between the Sunnis and Shi'ites in the country.
Since the internal political and military conflict in 2008, with the humiliation of the mainly Sunni March 14 Alliance by Hezbollah and its associates, it looked like the Lebanese Sunnis were finished.
The Shi'ites, because of their political and demographic strength, achieved a clear dominance. The underlying concern of recent events is that this balance may be shifting.
There are 1.5 million new Sunnis in the country. For a country with a population of less than five million, this is a major shift.
A number of articles in the Lebanese media this week have reflected the widespread sympathy felt among many Sunnis for the Nusra Front, which is widely felt in both Lebanon and Syria to be less extreme and more local in its orientation than Islamic State.
It is noteworthy that the Nusra Front mentioned the desire to avenge an affront against the refugees as the main goal of its Bekaa offensive.
All these topics point to a possibly emergent, new strategic challenge for Hezbollah – namely the emergence of a new, powerful, Sunni Islamist opponent, one possessing some popular legitimacy, considerable military ability and a capacity to operate across borders.
Hezbollah appears to be aware of this threat and is currently attempting to formulate its response to it. This is a new and emerging front in the sectarian war that has already consumed Syria and Iraq. It remains to be seen if the Shi'ite Islamists of Lebanon will succeed in resisting the challenge from their Sunni opponents.
Jonathan Spyer is a senior research fellow at the Global Research in International Affairs (GLORIA) Center and a fellow at the Middle East Forum.