1. Actually, the Just War Theory says a lot more than that
Augustine of Hippo, circa 400, considered the question, "Under what criteria can war be morally justified?" He drew up a series of criteria, called "Just War Theory," all of which must be satisfied to make a morally acceptable war. The first set covers the declaration of war, or jus ad bellum.
• Legitimate Authority. Essentially, only a legitimate government may declare war. This disqualifies revolutionaries or terrorists. Whenever I have mentioned this in a class, at least one student will say that this one means that the American Revolution was not a just war. I quote Patrick Henry, "If this be treason, let us make the most of it" and Benjamin Franklin's "We must all hang together, or most assuredly we will all hang separately."
• Just Cause. This disallows war for economic gain, land seizure or strategic position. Self defense is generally accepted as a just cause.
• Proportionality. For Sylvania to go to war with Freedonia because of a minor violation would be unjust. Consider the War of Jenkins' Ear.
• Probability of Success. You cannot lead your country into a war it has no chance of winning. National suicide is inherently unjust.
• Last Resort. Every possible effort should be made to settle differences without war.
After the war starts, it must be conducted justly, jus in bello.
• Discrimination. The acts of war should be directed towards the inflictors of the wrong, and not towards civilians caught in the middle. Prohibited acts include bombing civilian areas with no legitimate military targets. Acts of terrorism or reprisal against ordinary civilians. Many believe that weapons of mass destruction for any reason (such as the use of an atomic bomb) are forbidden.
• Proportionality. The force used must be proportional to the wrong committed, and to any possible good outcome.
• Minimum Force. Excessive and unnecessary death and destruction must be avoided. It differs from proportionality because the force proportionate to the goal may exceed the force necessary to accomplish it.
Brian Orend, from the University of Waterloo, Ontario, wrote an article, "Justice after War". This article suggests an additional set of criteria governing the ending of a war and the immediate post-war period, jus post bellum.
• Just cause for termination. A state should end a war if there has been a reasonable vindication of the rights that were violated and if the aggressor is willing to negotiate. The terms of surrender may include a formal apology, compensation, war crimes trials and perhaps rehabilitation.
• Right intention. Revenge is not permitted. Also, the victor must investigate and punish war crimes committed by its own armed forces.
• Proper authority. The peace terms must be set out by a legitimate authority, and the terms accepted by a legitimate authority.
• Discrimination. The victor must differentiate between political and military leaders, and between combatants and civilians. Punitive measures are to be limited to those directly responsible for the war and its conduct.
• Proportionality. The surrender terms must be proportional to the wrongs that set off the war. Draconian measures, absolutionist crusades and attempts to deny the losers the right to participate in the world are not permitted.
This just touches the surface of what is actually a quite complex subject.
Incidentally, no war in history has met all the criteria for a just war.