The West and the Arab oil states have been supporting ISIS economically and through arms deliveries in order to weaken Assad. And even now that the Arab oil states are part of the US-led coalition, several powerful Arabs continue to sponsor ISIS, and the Arab governments are doing little to prevent them from doing so, again because they wish to weaken Assad. As for Turkey, their border with Syria and Iraq has been completely open, allowing Jihadists to pass through, whereas pro-coalition Kurdish fighters were, at one point, prevented from crossing. Also, the US by accident released a top Jihadist, who's now one of the leaders of ISIS.



Assad's regime has on purpose released leading ISIS leaders and refrained from attacking ISIS fighters in several incidents, and this in order to cause fragmentation in the opposition, weaken the FSA and give the West and the Arabs another problem to handle. Iran has supported ISIS for the same reasons, as well as in order to keep Iraq weak and under their influence.



Under Maliki's leadership in Iraq, Sunni Muslims have been neglected and treated unfairly, causing them to riot against him and allowing ISIS to gain more influence.