Exactly.
Should we not concern ourselves with crime in our cities and townships and instead just secure our homes?
If you do that, sooner or later you'll find yourself barricaded behind your own walls trying by yourself to fight off the barbarians at your gate, wishing you'd beaten them before they surrounded you.
And that, in a nutshell is the problem with Libertarian national defense policy. Rather than maintaining a healthy skepticism toward international involvement, it pretends instead that America has no legitimate interests beyond our borders worthy of military defense.
The primary purpose and most legitimate role of a national government is to protect its citizens against violent force, especially when it is initiated from beyond one's borders. Else, how is liberty possible?
America did not "create" ISIS by unnecessarily involving itself in the internal affairs of other nations, even if sometimes in history, we have done just that.
ISIS was created by radical Islamists who prevailed upon those we liberated from local tyrants we might have chosen to leave alone, and then cruelly abandoned them to far more dangerous transnational forces who will not ever leave
us alone.
