[L]ibertarian first principles of individual rights and the rule of law tell us little about what constitutes appropriate and effective self-defense after an attack.But they sure as hell tell us what constitutes inappropriate "self-defense" after an attack. Such as: don't commit mass murder, don't destroy a people's infrastructure so they will die of starvation and disease, and don't violate the rights of the people allegedly being defended. The principles also provide guidance in how to avoid attacks and the need for self-defense in the first place. Such as: Don't prop up and arm dictators, don't overthrow elected regimes, don't aid those who oppress others, don't go out of your way to acquire enemies, etc. etc. etc.
Come to think of it, a libertarian can get quite a lot of foreign-policy guidance from his first principles. If he's really interested in trying.