Libertarians are rightly skeptical of military interventions. A simple reason is that military interventions tend to do more harm than good. This simple reason was enough to justify opposition to the war in Iraq. Saddam Hussein was horrible, but the prospects of instability and civil war were always high.
And, indeed, intervention in Iraq did more harm than good. At least one hundred thousand Iraqis are dead, and the government that replaced Saddam is unstable and fractious, unable to maintain basic territorial integrity in certain areas. And now ISIS runs much of Iraq.
So when people suggest military intervention against ISIS, we have good reason to be skeptical. But I think we have less reason than usual.
The reason is that if we intervened against ISIS, we could probably destroy most of their organization, given that they are an essentially territorial movement. Further, and more importantly, while there will be blowback in the form of guerilla warfare, new terrorists, casualties, etc., it is hard to see how anyone worse would replace ISIS. If we “roll the dice” again with an intervention, even though the odds of coming out morally ahead are generally low, the odds of coming out morally ahead of having ISIS run parts of Iraq and Syria are probably pretty high.
Conquering ISIS with a multinational army seems more likely to do more good than harm than any other proposed intervention in my lifetime. I don’t support intervention. But I have a much harder time getting upset by the prospect.