Can you urge that side to go all-out for victory?  Murray Rothbard, the greatest of all libertarian theorists, did not think so. And this is true even if you have assessed the conflict correctly. Let's look at what he says in his great book The Ethics of Liberty.

As you might expect, Murray doesn't begin his analysis by taking conflicts between states as the starting point. He asks what individuals involved in a conflict could properly do in an anarcho-capitalist society. Here is what he says:

"Before considering inter-State actions, let us return for a moment to the pure libertarian stateless world where individuals and their hired private protection agencies strictly confine their use of violence to the defense of person and property against violence. Suppose that, in this world, Jones finds that he or his property is being aggressed against by Smith. lt is legitimate, as we have seen, for Jones to repel this invasion by the use of defensive violence. But, now we must ask: is it within the right of Jones to commit aggressive violence against innocent third parties in the course of his legitimate defense against Smith? Clearly the answer must be "No." For the rule prohibiting violence against the persons or property of innocent men is absolute; it holds regardless of the subjective motives for the aggression. lt is wrong, and criminal, to violate the property or person of another, even if one is a Robin Hood, or is starving, or is defending oneself against a third man's attack. We may understand and sympathize with the motives in many of these cases and extreme situations. We (or, rather, the victim or his heirs) may later mitigate the guilt if the criminal comes to trial for punishment, but we cannot evade the judgment that this aggression is still a criminal act, and one which the victim has every right to repel, by violence if necessary. In short, A aggresses against B because C is threatening, or aggressing against, A. We may understand C's "higher" culpability in this whole procedure, but we still label this aggression by A as a criminal act which B has every right to repel by violence. To be more concrete, if Jones finds that his property is being stolen by Smith, Jones has the right to repel him and try to catch him, but Jones has no right to repel him by bombing a building and murdering innocent people or to catch him by spraying machine gun fire into an innocent crowd. If he does this, he is as much (or more) a criminal aggressor as Smith is. The same criteria hold if Smith and Jones each have men on his side, i.e. if "war" breaks out between Smith and his henchmen and Jones and his bodyguards.