The tactics employed by the ISIS are so brutal that in February the main branch of Al-Qaeda in Syria disavowed the group entirely. The U.S. government, which has been funding and arming rebels in Syria has attempted to distance itself from both the ISIS and mainline Al-Qaeda fighters in Syria, however the so called "moderate" rebels that Washington is publicly supporting, have admitted that they regularly carry out joint operations with Al-Qaeda, and do not consider the group their enemy.
U.S. officials acknowledge that American weapons and vehicles have been diverted to extremists in both Iraq and Libya, but as of yet they have dodged the obvious questions related to their channeling of arms to insurgents in Syria.
Weapons and funding aside, it is the U.S. government's push to topple Assad that has made ISIS's rise to power possible. The group has thrived in the chaos of the Syrian war, and in its weakened state the Syrian government has been unable to push them out of their strongholds