This is a conundrum for the ideal non-interventionist libertarian state… would such a state have any moral duty to intervene in the affairs of another state if that state were carrying out genocide on its own people or enslaving a certain group that lived within its territorial boundaries? The problem is that we keep considering the state as a group of people, but it becomes clearer I think as to the proper course of action if we break it down to 3 people. If I happen to come upon Person A beating Person B and clearly Person B is the weaker person and is not able to defend themselves, do I have (a) a moral duty to help person B or a (b) civil duty to help person B. Most people would probably say yes to (a), however for (b) the correct response I think is “no”… because in so doing you could be endangering yourself, you don’t know what person A is carrying as a weapon or what they are capable of, and you do not know if person B even wants your help, maybe he doesn’t, maybe it’s all part of his master plan before he counter attacks, who knows, the point is you would be interfering and there is no non-arbitrary way to know if that interference is welcome or not… even asking might be interference… in other words one is not obligated to put themselves in harms way from a civil standpoint (morally however I think if we see someone at the point of death we’re going to do something as we would expect the same were we in that position).
Now, the situation changes if person B requests of me assistance, I now have become involved in the altercation and clearly person B is desirous of assistance. Because I have been asked it is necessary for me to make a choice and civilly either choice is acceptable… so from a civil standpoint we move from a definite “no” to new options of “yes” or “no” if asked. If asked and accepted then it is completely keeping with the non-aggression principal because I would not be aggressing… I have a right of association as does person B, and we are choosing to associate and participate in shared defense… just as a family defending their home… any member may defend against an aggressor if that aggressor attacks any member of the family not only if they happen to be under attack themselves (i.e. I would not let someone attack my son just because they aren’t aggressing me and say “well it’s up to him to defend himself”).
So, if we now extend this from 3 people to thousand or millions, we can argue that if the Jews during the holocaust had asked for US assistance the US government could have legitimately considered the request and chosen to help or not. But strictly the makeup of the military would have to be different, it would have to be all volunteer and before every mission it is explained what the purpose of the mission is and only those soldiers wishing to engage in that mission would go. If it is a genuine attack on US soil you would not find one soldier not opting to join… but a situation like Vietnam you would have had far fewer.
So that is my long winded explanation of how I see libertarian principals being applied to the riddle of state non-interventionism. Right of association applied to co-defense and individual voluntary ascent to the request allows for groups of individuals to crush “evil”… since as we saw in WWII and in other arenas (killing fields of Cambodia) evil does unfortunately still exist and we libertarians don’t want to get mischaracterized as uncaring because we would have just allowed such atrocities to occur while painting ourselves into the “non-agression/non-interventionist” corner.