War is perfectly justified.
They have something we don't and we want it. They refused to give/sell/lend it to us. Let's go!
Oh dear, we have an overpopulation problem. Send in the conscripts!
We think they're harboring something bad. Peacekeeping action in 3... 2... 1...
Hey look, we can take this piece of whatever and everyone's powerless to stop us. 'Ere we go!
If you take the Revolutionary War and say "Was it Justified?" I would say no. If you took World War 2 and said the same question, then I would say Yes.
The Revolutionary war was not justified. Why? Because Since There was no serious Hate crimes or Genocide, the British were NOT the Aggressor in the war. They were simply Political tyrants. There was Abuse and some Accidental Deaths, but all in all the Americans were the aggressors. (The deaths being Boston Massacre, in which was simply A protest gone ugly.) The British Simply Felt threatened by the Americans stockpiling weapons (I think in Lexington or Concord.) So they went to seize them, hence the war starting when the "Shot heard round the world." happened.
Overall The Revolutionary war was Unjust.
Take World War 2, though. The Americans were attacked. (Pearl Harbor.) We declared war in the state of self defense. We did NOT want to get bombed again. It was self defense, so that seemed justified.
In all, I think it is a matter of perspective, but deep down I think no war is justified. Maybe World War 2 was justified in the eyes of the Allies. I still am a complete maybe on this.