Well. perhaps the answer is why the USA & EU were keener to go for regime change than to fight ISIL. Geopolitic.
I think USA & EU think ISIL not real danger... Too extremist, it will collapse by itself, and still in need of Western Technology/medicament/goods, so it will tamed itself to become an "Saudi Arabia", potentially. Not a place to leave yourself, but you can trade with. So, they focused on letting ISIL to grab as much as lands and population to enslave as possible, then wait for the change. But Assad was on the way, so, they aimed at him. A little bit of bombing ISIL (not too much), a bit a money and weapons to "moderate" opposition they knew will end in ISIL hands, looking the other way when Crucifixion, burning people alive and women sold to slavery, campaigning against destruction of all temples, well, routine.
Then Putin racked the game. Bombing and helping Syrian Army, not good. Even worst, look that he got result.
And just after the public admission that there is in fact no more moderate opposition, and 1,000,000 refugees who had enough to wait to be sold to ISIL by USA & EU lack of real action (as it was not the plan anyway), walking into Europe.
So whom Putin is bombing?
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