There are two factions in a conflict. A third-party may choose to intervene by supporting one of the factions. We consider a third-party who maximizes a weighted sum of the welfare of the warring factions and the non-combatant population. In the case of a nonmilitary intervention, we obtain the following results: if the third-party cares equally about the warring factions and the rest of the population, then he will not intervene. If the third-party cares more about the warring factions, then he might intervene and will help the stronger faction unless he places a sufficiently higher weight on the welfare of the weaker faction. The stronger faction is able to appropriate more resources from the rest of the population. However, we find that helping the stronger faction might make the rest of the population better off, since this reduces the aggregate cost of conflict. On efficiency grounds, helping the weaker faction is optimal if success by the weaker faction preserves the rule of law, respect for private property leading to higher output. We also find that the third party is likely to intervene if success in the conflict is extremely sensitive to effort. In the case of military intervention, we find that the third-party will intervene if he cares sufficiently about the rest of the population or cares about the net resources that will be left after the war. We present examples where the third-party chooses military intervention over non-military intervention and vice-versa.
