The US-led coalition that now appear as though they are fighting ISIS knowingly aided the rise of the Islamic State for the purpose of isolating Assad and combating expanding Iranian influence.
At least as far back as August of 2012 the very same anti-IS coalition knew full well that the precursors to ISIS, al-Qaeda in Iraq (AQI) and Islamic State in Iraq (ISI), dominated the Syrian opposition along with other al-Qaeda affiliated groups. They knew that AQI was declining during 2009-10, yet was resurrected due to the insurgency in Syria. In spite of this, the US and her allies continued to provide aid, funding, weaponry, and training to these same extremist groups, specifically seeing their rise (and the horrendous crimes against humanity that they partook in) as a strategic asset for their geopolitical aims.
The rise of the Islamic State was not only predicted, it was the expressed aim of the powers sponsoring the sectarian Syrian opposition for the purpose of opposing Assad and containing Iran. Despite the fact that the rise of an Islamic State was predicted to have dire consequences for Iraq, including the fall of Mosul and Ramadi, support from the US-coalition to the Syrian opposition continued to manifest, leading to the conclusion that this was either the expressed intent, or an accepted byproduct of these policy decisions.