and over 1B$ in assets, that is utterly ruthless and vows to destroy civilization as we know it and is growing by leaps and bounds. It has a well trained and well equipped military force and NO one with the power to stop it that has the will. Half of their fighters are from elsewhere and will return as trained terrorists. Millions of Muslims support them and the West not only let it happen but they are willing, so far, to do little to stop it. Even the Pope gets that violence must be used to stop them.

http://www.foxnews.com/world/2014/08/21/isis-thirst-for-oil-could-lead-to-global-catastrophe-if-unchecked-experts-say/ 

They have total assets that are only 1/4 the cost of a single USN destroyer. Can they do damage ? Certainly. But they stand ZERO chance of winning any sustained conflict against the west. All they can do is resort to terror.

Our response to them could be potentially much more damaging to our society than they ever could be. We could cripple our economy, empty our treasury, make our nation completely and irrevocably insolvent, permanently curtail our civil liberties and make ourselves a police state etc.

They can't destroy us. But how we respond to them very well could.