Once they are face to face, they could stop and go back to their old ways of "don't fight me, I won't fight you." But clashes are more likely, mostly in Aleppo. And if at some point ISIS wants to move north towards Idlib and maybe Hama, there could be clashes between the two in that area as well.
Syria Deeply: What happens to non-ISIS rebel groups as focus shifts to the ISIS-regime fight?
Joshua Landis: What we're moving towards, ultimately, is going to be a contest between the Assad regime and ISIS. The rebel militias that have been extraordinarily fragmented and incapable of uniting are going to get squeezed out by these two very brutal but quite capable forces of ISIS and the regime.
By all accounts the Obama administration is pulling its hair out over the question of who to back in Syria, and whether the present policy of backing the moderate rebels is a staying policy. There's a lot to suggest that it's not. Because for the last three years the stalemate has zapped the power of both the rebels and the regime, creating a power vacuum in the east. And it's there that ISIS has been able to incubate without facing any determined opponent.
The opposition's inability to unite, its constant backbiting – it's dizzying trying to keep track of who is aligned with whom. We're seeing this now in Ghouta, where Jabhat al-Nusra and the Islamic Front seem to be at daggers drawn.
And in Aleppo, they've been incapable of uniting against ISIS. They're being picked off one after the other. Assad's main enemy in Aleppo and Damascus – the areas he truly still cares about securing – are these fractious rebel groups. That's why he hasn't been going after ISIS.
So now what does Obama do about Syria? Increasingly people are beginning to say, You have to side with him, or you at least have to not undermine him by supporting the rebels.
Syria Deeply: With whom will Arab and Western powers side, ISIS or the regime?
Landis: Assad and ISIS are ideological enemies, but they have strategic common interests – they're both busy attacking rebel forces. ISIS has left Assad alone for strategic reasons, which is that it's been busy attacking weaker forces. But the showdown is coming, and regional forces will choose to back Assad. ISIS doesn't have friends there.
Assad is going to get friends out of this – the Syrian conflict has become infinitely more simple today than it was even five months ago for many world powers, because now it seems to be a simple struggle between ISIS and Assad. It's hard to see anyone in the Western world choosing ISIS over Assad.
This article originally appeared on Syria Deeply.