I have had a few requests to explain more clearly whether I thought that the American Revolution or the Iraq war were just wars. I'll take a shot at it, although I know some of my readers will sharply disagree no matter what opinion I give.
I'm going to analyze a very narrow question so I can be more clear. I'm going to explain what just war theory says, and then try to place a few wars (including the Iraq War and the American Revolution) on some sort of "justice scale." On one end of the scale would be totally unjust wars of aggression, such as Saddam's invasion of Kuwait or Germany's invasion of Poland. On the other end of the scale would be wars as close to just as possible, such as (perhaps) England's defense of European allies in WWII. Then we can see where other wars might fit it.
Just war has two sets of criteria. The first set describes just causes for going to war (jus ad bellum) and the second set describe conduct in the war (jus in bello). What are just causes for going to war? Just war theorists usually break down jus ad bellum into multiple criteria, but I'll simplify a little.
1) War is justified in response to aggression - to reverse the aggression and to retaliate. While the individual may not be right to retaliate, a government probably has an obligation to retaliate. A government must retaliate against criminals in order to defend the innocent, and thus it must retaliate against aggressive actions from other countries. A government may also be obligated to respond with force when an ally or an innocent people group is targeted by an unjust war. Thus, the US-led defense of Kurds from Iraqi genocide in the 1990s was a just use of force.
2) War for economic gain is not justified - this consitutes a war of aggression. What makes this criteria complicated is that sometimes a nation may be justified in going to war (defense of an innocent nation), but the reality is that some economic benefit may result. Further, some government leaders may be motivated by just causes, while others may be motivated by economic causes.
3) Only legitimate authorities may begin a war. In other words, a group of independent US citizens cannot decide to attack Iran, even if other grounds for a just war have been satisfied.
4) The war's goals must be limited to responding to the wrong suffered. For example, reasonable goals during the first Gulf War were to expel the Iraqi army from Kuwait and destroy a significant part of its army so that Iraq could not attack again. Overthrowing the government of Iraq or invading Iraq would have gone beyond reversing the wrong and appropriate retaliation (at least, that was the reason why the elder George Bush chose not to invade Iraq in 1991). This concept is related to criminal justice. We give fines for speeding and increasing amounts of jailtime for more serious offenses. In war, the intended response should also be limited by some concept of justice.
5) Most important criterion: war is just only if it is the last resort. A nation must demonstrate that it has taken every reasonable action short of war. This is based on the idea that war is never good, and always goes worse than intended. War may be necessary and therefore just, but it should be avoided if there is any other reasonable solution.
6) Reasonable chance of success. If a nation has no hope of winning, then by going to war it is wasting lives.
Next post: Jus in bello - just practices in war.
By the way, some Christians are pacifists - they believe that war is never justified or never necessary. I will not cover that approach in these posts, but you can learn more about their approach at the Just Peacemaking website.