I'm really torn on any military involvement now in the Middle East, in part because the involvement of the past ten years hasn't worked very well (why didn't Bush & Co do a better job of planning for after the war?), and in large part because I don't trust Obama to not make things worse. It's his trademark.
That said, ISIS has killed Americans. Whether they deserved it because they were in the enemy's back yard can be debated. I don't know if they really have the capability to reach US shores, but I'm sure they have the capability to cause trouble for us elsewhere in the world. By trouble, I mean death for Americans.
I learned something recently that is related to this debate. I knew that, in the early years of Hitler's buildup of the German military, the armies of France and Great Britain were large enough that either country could have taken on the Germans and forced Hitler to stop expanding his military. I didn't know that, when the Germans put troops along the Rhine in 1936, the officers were under orders to retreat at the first sign of the French army. The French never sent their army, though, figuring that it would provoke the Germans.
Inaction on our part will embolden ISIS. I think there's no doubt about that. The question is what their capability will be in the next few years, not what it is now.