I'm surprised to see so little discussion of fear as a motivator. If we define war as ""morally acceptable collective violence by one group against another group", as above, we have to ask what makes violence morally acceptable. The answer, usually, is a perception of threat, whether justified or not. When leaders try to rally the populace to war, the first thing they try to do is make them afraid, to position the proposed antagonist as a threat. That's typically not been difficult: there's a long human tradition of fearing "them"... the other, the different, those who aren't "us".

Once fear is in play and the perception of threat is entrenched, presenting violence as a morally acceptable option becomes a lot easier. Attacking another group to steal their $#!t and rape their women makes us feel sleazy; killing to protect our own $#!t and our own women makes us feel noble... and of course once violence is in play, if an opportunity for a bit of plunder comes along, to the victor belong the spoils.