To be honest, engwell, that is an opinion, not the "only truth".

ISIS wasn't squat when Coalition combat ops ended in Iraq. It didn't become a problem until the Syrian civil war when it became a proxy for its Sunni supporters in Turkey and the Arab world and began enjoying the battlefield successes that have enabled it to seize territory, huge amounts of weaponry and valuable revenue resources, such as oilfields. Another factor was the the willingness of the international community to ignore the rise of ISIS as long as the al-Maliki government remained in power, and by the time he left cities like Mosul, Fallujah and Takrit were already in ISIS's hands. Then there is the issue of the ineffectiveness and/or cowardice of the Iraqi and Syrian armies, who are notoriously awful and have been exposed as such again by ISIS.

Then, of course, there are the ideological matters that dixon mentioned and figure into the rise of any and all jihadist groups. The "caliphate" dimension of the Islamic State and its remarkably sophisticated propaganda have also aided greatly its recruitment in ways that its predecessors in al Qaeda, et al, were unable to exploit.