Riedel: The Assad government would see this as creating a string of Shia pearls – Hezbollah and Assad in a rump Syria, and Maliki in a rump Iraq. The interesting question is, How expensive it is going to be to maintain three client states for Tehran over the long term? The Syrian state doesn't produce any income, so it's a net loss. Lebanon may be not that much of a burden, and the Iraqi state should theoretically be able to pay for itself with all that oil. But that depends on whether the parts work. Iran could find itself in a position where it's fighting off an awful lot to pay for.
Karasik: Iran has been the Syrian government's number one supporter, and Tehran has kept Assad in power because it put so much effort into supporting the Syrian military with Iranian Revolutionary Guards on the ground and also moral and financial support. For Iran, and for Syria, this is an important moment that will test the future of their relationship.
Nerguizian: Iran is literally [making] an attempt at changing the balance of power in Iraq in a way that undermines what Iran views as a hegemonic interest. For better or worse, Baghdad and eastern Iraq, at a minimum – to say nothing of the greater Levant – they all fall into what the Iranians view as the "near abroad" in their sphere of influence. They're not going to allow for a scenario where somehow Baghdad is going to dramatically fall into the hands of ISIS and ... there comes a point where the advance becomes more difficult or less sustainable.
ISIS is not a modern military machine, and granted, the Iraqi government and forces are not the definition of an effective military force. But what you do see, whether it's Syria or Iraq, is that forces that are fighting this, especially when the scale is broadly existential or the outcome guarantees a real loss of control, tend to harden pretty quickly – especially in areas that matter. So if you have commanders that were ineffective, they won't have a leading role to play in fighting back against ISIS. This is similar to the Syrian military, where you've seen a weeding out of elements that are not loyal enough, or loyal but not capable, or who have just flipped sides.