Due to the Israeli bombings in Syria on the day of the attack Assad and a majority of government figures are killed when Damascus was nuked. The nuclear winter only exacerbated the long term drought that started the civil war in the first place. ISIS has already sprung up before the war and had mostly been confined to rural sectors of Iraq and Syria with the main exception being the Iraqi city of Mosul, being the largest occupied city and one of the most strategic holdings. It became even more strategic after the war because while Syria suffered a few nuclear detonations no such weapons detonated in Iraq despite US and other western military presence.
While the Iraq war ended in 2011 American and other NATO troops came back in early 2014 when Iraq was dealing with its own Civil War. It is speculated that Russia did not target their military positions in Iraq because they wanted to save their weapons for more strategic targets in Europe and North America. In the weeks following the war, starving survivors make their way to the bases as drought affects Iraq too and any food imports had abruptly ceased. As desperation grew some bases began to open fire but that was a mistake because their ammo was finite and due to the collapse of supply lines no more were coming. The bases also had a finite amount of food and would eventually be in the same situation as the masses surrounding them. So many bases evacuated, using the last of the fuel in any aircraft or vehicles they had. Some were killed by the masses while others fled in a general direction towards Australia, which still had a functioning government.
With the desperate populace and a withdrawal of any force that could hamper them ISIS attempted a coup in both Iraq and Syria. Many civilians welcomed them with open arms too because of the hardships caused by the nuclear war turning many towards religion even stronger than they were before, as religion is very popular immediately after a crisis. By early 2015 ISIS has complete control over the Fertile Crescent and is waiting for radiation to die down in Israel and Turkey before expanding too far North and West. Iran is also on the brink of collapse, pretty much leaving them with unadulterated influence in the Middle East.