Re: only if they lost ...Jus ad bellum
Just cause The reason for going to war needs to be just and cannot therefore be solely for recapturing things taken or punishing people who have done wrong; innocent life must be in imminent danger and intervention must be to protect life. A contemporary view of just cause was expressed in 1993 when the US Catholic Conference said: "Force may be used only to correct a grave, public evil, i.e., aggression or massive violation of the basic human rights of whole populations." Comparative justice While there may be rights and wrongs on all sides of a conflict, to override the presumption against the use of force, the injustice suffered by one party must significantly outweigh that suffered by the other. Some theorists such as Brian Orend omit this term, seeing it as fertile ground for exploitation by bellicose regimes. Legitimate authority Only duly constituted public authorities may wage war. Right intention Force may be used only in a truly just cause and solely for that purpose—correcting a suffered wrong is considered a right intention, while material gain or maintaining economies is not. Probability of success Arms may not be used in a futile cause or in a case where disproportionate measures are required to achieve success; Last resort Force may be used only after all peaceful and viable alternatives have been seriously tried and exhausted or are clearly not practical. It may be clear that the other side is using negotiations as a delaying tactic and will not make meaningful concessions. Proportionality The anticipated benefits of waging a war must be proportionate to its expected evils or harms. This principle is also known as the principle of macro-proportionality, so as to distinguish it from the jus in bello principle of proportionality.
Jus in bello Once war has begun, just war theory also directs how combatants are to act: (Jus in bello)