The Western distaste for IS brutality and immorality should not obfuscate strategic clarity. IS are truly bad guys, but few of their opponents are much better. Allowing bad guys to kill bad guys sounds very cynical, but it is useful and even moral to do so if it keeps the bad guys busy and less able to harm the good guys. The Hobbesian reality of the Middle East does not always present a neat moral choice.
The American administration does not appear capable of recognizing the fact that IS can be a useful tool in undermining Tehran’s ambitious plan for domination of the Middle East.