Moreover, the US supported them and played an active role in bringing them to power.
The Obama administration approved a secret CIA program called Timber Sycamore, under which the Americans began training and arming militants fighting against the Syrian army. Some of the weapons supplied through this program were redirected to ISIS, which U.S. forces later led a coalition to fight against (a coalition that now includes Syria).
All in all, the U.S. spent a billion dollars on this program.