Your text has a disturbing lack of sarcasm and absurdity. You shouldn't do it again.
They wouldn't need to do that. If one rebel front collapses, IS can just fill in from their front. Of course, a fight between other rebel groups and Assad will weaken both, leaving an easier battle for IS against whoever wins.
It's relatively simple: if your goal is to destroy IS, start by attacking IS. If your goal is to keep Assad in power, bombing any rebel group will do. If you destroy all other groups than IS, other countries will have no other forces to back against IS than Assad.
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