Let me add one point.
If the U. S. refuses to be the agent of change in instances like this, sooner or later another nation will start to take over resolving these sorts of situations.
Will you honestly agree that it is better if the U. N., Russia, or China become that agent?
Do you really want these or another entity to replace us as the world’s conscience?
We either engage, or someone else eventually will. And this will mean they had to gear up to do things like this, and have become a greater threat to us in the process.
Frankly, at this juncture I think Russia would be better at handling thugs like ISIS, than we would.