- The West is psychologically unequipped to deal with this. You can hear the total confusion in the Obama Administration's response: "We urge all parties to the conflict to allow safe access to the United Nations and its partners so they can deliver lifesaving humanitarian assistance."
- They are capturing territory, but not people. By killing or driving out all opposition, they make a guerrilla campaign almost impossible. There's no opposition left for us to arm.
- Refugees make troop movements difficult. Roads are clogged, cities overwhelmed, transportation assets oversubscribed, etc. If you want to move a mechanized brigade from Point A to Point B and the roads are full of desperate refugees, good luck with that.
- Refugees tax you, not them. ISIS is left with resources made for a much larger population once they finish killing a bunch and driving out the rest. Supply problems for ISIS? I don't think so.
- ISIS is looting every place they capture and using that to fund the next offensive.
- Without uniforms, ISIS has neutralized air power. Just what are you going to bomb? Since there are no opponents left alive behind their lines, you have no hope of recruiting spotters to guide your air strikes.
- ISIS is quite simply more fearlessly brutal than everyone else and their opponents can't deal with it.
Dig this video from the fall of France in 1940. Fast forward to 5:30 to see troops mingling with refugees. Add a couple of car bombs and snipers in your imagination to get some real excitement.
At the same time, ISIS has made the solution trivially simple. Since there are no friendlies left in ISIS-held territories, everything is now a target. No rules of engagement needed, simply obliterate everything and kill everyone in ISIS territory.
Anyone want to take some bets on whether or not the West has the stomach for that kind of war?