Jaysh al-Muhajireen wa al-Ansar was arguably the largest contingent of the 'third way' Jabhat Ansar al-Din jihadi coalition of northern Syria, having absorbed a component of the coalition- the Green Battalion- last year. Linked to the Caucasus Emirate through its amir Salah ad-Din al-Shishani, Jaysh al-Muhajireen wa al-Ansar had been reluctant to advocate openly denouncing and confronting the Islamic State [IS] on account of an anti-fitna stance, even though such a position is not tenable in the face of IS' demands for allegiance. The other members of Jabhat Ansar al-Din- Harakat Fajr al-Sham al-Islamiya and Harakat Sham al-Islam- lambasted IS at length in separate statements (see here and here) following its renewed offensive on rebel-held areas of north Aleppo countryside beginning at the end of May 2015. Harakat Fajr al-Sham al-Islamiya had acknowledged the prior stance of neutrality regarding IS.
Jabhat Ansar al-Din as a coalition- likely deferring to Jaysh al-Muhajireen wa al-Ansar's insistence on an anti-fitna stance- initially did not promote these criticisms of IS, but it eventually acknowledged IS as a hostile problem comparable to the "Nusayri regime", PKK and the U.S.-led coalition in a fatwa on Turkish plans for intervention in northern Syria in August 2015, also mentioning IS' "extremism, criminality and corruption." In short, the fatwa, which is quite elaborate, disapproves of the planned Turkish intervention but accepts that others who are 'sadiqeen' ('truthful/honest') might disagree.
In any case, prior to that fatwa, Salah ad-Din al-Shishani had already been removed from his position as head of Jaysh al-Muhajireen wa al-Ansar in June 2015, going on to found his own Caucasus Emirate-aligned group in Syria. I agree with the line of analysis that this development seems to have helped pave the way for Jaysh al-Muhajireen wa al-Ansar to join Jabhat al-Nusra, Syria's al-Qaeda affiliate. Indeed, at this stage, as Joanna Paraszczuk highlights, Jaysh al-Muhajireen wa al-Ansar is in fact mostly Arab in composition and led by a Saudi (Mu'atassim al-Madani), who replaced the Tajik amir who had taken over after Salah ad-Din al-Shishani's removal.
What next for Jabhat Ansar al-Din then? At present, there is no sign it will follow Jaysh al-Muhajireen wa al-Ansar's lead in joining Jabhat al-Nusra. Ali Saber, a Jabhat Ansar al-Din media office activist linked to Harakat Fajr al-Sham al-Islamiya, tweeted on 22 September (below): "Jabhat Ansar al-Din will continue its jihadi activity and now comprises Harakat Fajr al-Sham al-Islamiya and Harakat Sham al-Islam," while wishing Jaysh al-Muhajireen wa al-Ansar all the best in its pledge of allegiance.
Similarly, the leader of Harakat Fajr al-Sham al-Islamiya told me via direct message on Twitter (below): "No, we don't have a plan/project to join Nusra." This is so despite the fact that Harakat Fajr al-Sham al-Islamiya has previously held a joint da'wa campaign with Jabhat al-Nusra in Aleppo denouncing democracy as the "religion of the West" and asserting that rule belongs to God alone.
Perhaps the other components of Jabhat Ansar al-Din may eventually come around and merge, but it is unlikely to strengthen the rebel efforts against IS in north Aleppo countryside, seeing as how most Jabhat al-Nusra fighters have already pulled out in anticipation of a U.S.-Turkish safe zone, such that only Jabhat al-Nusra members who are local to the north Aleppo countryside remain on the frontlines as a minor portion of the rebel forces resisting IS. Indeed, it is notable in this context that the Jaysh al-Muhajireen wa al-Ansar allegiance statement makes no reference at all to the IS problem as a reason to unite with Jabhat al-Nusra, but instead keeps with its traditional anti-fitna stance in focusing on non-Muslim enemies: the Assad regime and its Shi'a and Russian allies as well as the 'Crusader' West. No local evidence suggests any meaningful participation of Jaysh al-Muhajireen wa al-Ansar in fighting against IS in north Aleppo.
Below is the allegiance statement of Jaysh al-Muhajireen wa al-Ansar translated by me:
"In the name of God, the Compassionate, the Merciful
Praise be to God the Lord of the Worlds and the most preferred prayers and most fulfilled approval be upon the one sent as a mercy to the Worlds- our Prophet Muhammad- and all his family and companions. As for what follows:
The one who follows the arena of al-Sham sees the size and ferocity of the war on our people- the noble people of al-Sham- and the gathering of ranks of the people of kufr [disbelief] for their war, comprising Nusayris [Alawites], Rafidites [Shi'a], Russians and Crusaders. So this scenario pushes us to unity, holding together and closing ranks in answer to the Almighty's words- "And cling fast to God's rope entirely and do not separate" [Qur'an 3:103]- as well as the Almighty's words: "Indeed God loves those who fight in His path in a row as though they were an edifice brought together" [Qur'an 61:4].
From this premise, we in Jaysh al-Muhajireen wa al-Ansar announce our allegiance to Jabhat al-Nusra, uniting for discourse, closing ranks, strengthening the power of the mujahideen, and vexing the enemies of the religion.
In this context we laud the rest of our brothers from the components of Jabhat Ansar al-Din with whom we gathered in unity of creed and project, and we ask God to bless them in their jihadi path and to grant them and the rest of the honest groups success to be a brick in building the Islamic project on the land of al-Sham.
And we ask God for success and what is right.
Jaysh al-Muhajireen wa al-Ansar
9 Dhu al-Hijja 1436 AH: 23 September 2015."