Over at Bleeding Heart Libertarians, Fernando Teson contends (convincingly, I think) that libertarianism is concerned with a defense of self and others, but of persons as such and not the state.
If so, the possible support for defenses of foreign persons increases:
The distinction between defensive and offensive wars is misleading because it treats the state as a “person” who can be Attacker or Victim. But states are not persons. When we (correctly) disaggregate the state, what we have is a group of human beings unjustly attacking another group of human beings. A defense of the victim here is not an offensive war: it is a defensive war, a war in defense of unjustly-attacked persons. As such, it should not be banned by the libertarian principle that condemns the offensive use of violence.
Well worth reading in full.