At least two of the twitter accounts I follow have been reporting on the recent events happening in Arsal, Lebanon. According to the two pictures posted above, the picture apparently shows Nusra and IS members manning a checkpoint in Arsal. While many news outlets are reporting IS fighters in Arsal, it should be noted that Jabhat al-Nusra has a much larger presence in Lebanon, as well as in Qalamoon (a region in Syria near the Lebanese border). Most of IS operations appear to be located in Raqqa--as seen with recent military gains there--as well as in Deir az-Zour and in Syrian Kurdistan.
Nusra, on the other hand, heavily operates in this part of Syria. On the Lebanese side, they have an official branch based there according to the US State Department. Not only that, they maintain a close relationship with theLebanese-based al-Qaeda group Abdullah Azzam Brigades;both groups have launched joint assaults on Lebanese and Hezbollah targets. The Abdullah Azzam Brigades has also launched assaults on Iranian targets inside Lebanon. In the same designation of the Lebanese branch of Nusra, the US State Department also notes that this branch has been in contact with and has close ties to another Lebanese group Fatah al-Islam. Unsurprisingly, members of Fatah al-Islam has died fighting alongside Nusra in Syria. However, the Islamic State has also launched some suicide attacks in Lebanon, but appears the majority are perpetrated by Nusra or other al-Qaeda groups.
What this could mean:
If these twitter posts are indeed true, this is both intriguing and interesting considering the two groups are fighting each other in other fronts of Syria. I would assume the Islamic State is cooperating with Nusra for a few reasons.
1. By working with Nusra, it could gain a larger footprint inside Lebanon. This would help it to attract new recruits (with Sunnis in Lebanon having some sympathies to the IS), as well as possibly opening up a new front for them in the Lebanon.
2. It could help it to achieve its goal of capturing territory inside other states in the Levant.
3. It would be a propaganda boost for them if they were to take over areas and towns in Lebanon.
On the Nusra side, I'm sure extra manpower is something they did not want to turn down. In fighting a common enemy, i.e Hezbollah, they probably realized the benefits of having an allied force with the stature of the IS in an assault of a Lebanese town. It is yet to be confirmed if this is the start of a larger battle against Hezbollah, but the inclusion of the IS signals that they might have expected a large fight between themselves and the Lebanese military accompanied by Hezbollah. If this does turn into a larger fight between Nusra and Hezbollah, having more allies would definitely be advantageous for Nusra.
It will be interesting to see if this temporary alliance blossoms into something stronger or remains temporary and eventually falling back into fighting each other. If the two were to hypothetically reach similar agreements elsewhere in Syria, or even full reconciliation, the landscape of the Syrian Civil War would be drastically changed. Indeed, this level of cooperation between the two, if it were to even happen, would literally be a game-changer inside Syria.
However, for now it appears this "truce" is only located in Arsal and Qalamoon. Only time will tell if this manifests into something bigger for the two groups or revert back to bitter infighting.