There's a difference between fighting a skirmish, a battle or a war. The first is always tactical, the second either or both of tactical or strategic, and the latter is always strategic or in the worst case, existential. We're not fighting a war against ISIS for the simple reason that they don't represent an existential threat to us. They represent an existential threat to the countries they operate in, and a longer term strategic threat to our interests in the region, but also are not a strategic threat to our security. That ignores the very occasional act of terrorism that takes place on US soil, but as others have pointed out, we have grown more of our own terrorists than we have imported.
You asked how many American lives would it be worth to lose to protect the threat we feel from them? I think now it's precious few because the countries under attack have every ability to marshal the resources to protect themselves and attack ISIS that ISIS does to attack them. For a long list of reasons they seem almost paralyzed to do that on their own, so we're giving them assistance. But the same forces that render them incompetent also mean they can't sustain the effort on their own after some initial successes.
An important question is to what extent is that also our problem. If they can't defend themselves, then maybe their ways of governing themselves are so thoroughly rotten to the core that there's not really anything there to save. That was our supposed mistake in helping to overthrow Qaddafi. After he was gone there was nothing and is still nothing. Our blind spot has always been to believe that with just enough help every country will want to be like us and will rise to the challenge that we did 240 years ago.
I don't think that's going to happen for many generations to come in Iraq, Libya, Egypt, Syria or Afghanistan, among others. Other countries, like Turkey, are beginning to lose their grip on the reality and dream of the secular state and may join them in chaotic and despotic rule sooner than we would have thought only a few years ago.
We have fought wars in the Mideast for almost 100 years for what amounts to a single reason, to protect the supply of oil that fuels industry and commerce in Europe and the US. We don't need that nearly as much as we did a decade or two ago, so if you were a 20 year old military recruit today sent overseas to fight in that part of the world, what would you be fighting for and would you be willing to die for it?