In general, people go to war because either they have something to gain, because the alternative is ultimately worse than going to war, or because there is no alternative.

...I could probably make you a pretty good map, a visual approximation, with some napkins and the cups and so on. [Armin rubs his chin thoughtfully, a habit he hasn't realized he picked up from his grandfather.] I could talk about territory, and resources, and what happens to people who are either, over a long period of time or quite suddenly faced with a deficit of both. But you're so smart, do you really need me to explain it to you in that level of detail? [Armin gives the most angelic smile. It's his very best 'don't take me seriously I'm just a kid' smile, when before he was so serious.] I'm sure you don't lack the imagination... Which means for me to go on about it is sort of a waste of both our time, don't you think?