how do you "crush" and or stop something that has no home base? That's the problem. The more you pound them into the sand (literally and figuratively) in Iraq, Syria, etc the more their message grows wings and takes flight and spreads. If you disperse them in one place, they grow bigger and stronger in other countries (NOT in the Middle East for that matter). This is an ideology we're trying to win a war against -- not a sovereign nation.
That's true, you can't defeat an ideology like you can defeat a military force.... but destroying any significant military force is a good start. You have to take away the areas where they can freely train, stage and wage war on other nearby areas as well as defeating the ideology. In fact, I'm not sure you can ever defeat the ideology anyway, you can just try to keep it out of your country as much as possible.