Yes, nations (or at least their leaders) go to war mainly for the reason of ignorance (especially an unawareness of their long-term interests) and institutionalized greed. But they justify to those who must die to implement the leaders' decision on the grounds of the enemy's quality of "evil." (what was Reagan's justificatory phrase, "The Axis of Evil"?).
I agree that it would be far more difficult to prosecute a war against others on the grounds of their ignorance and greed.
actually, thats arguable. leaders go to war because they are being attacked, instigated, insulted, denied something they want. and yes, sometimes greed. (i think that the worlds past that, though. i could be wrong) they keep fighting because the alternative is unbearable (slavery, oppression, being removed from power) (its usually the third one with dictators), or because they think they stand a chance. citizens go to war because they are either drafted, have lots of patriotism, or because they have no where else to go.