It's very easy to do. All you have to do is tell the stupid, poor frightened people that if they don't fight, the other side is going to march in, take what you have, and destroy your culture. And if you don't have anything, they will just rape and kill you instead.
So, now you have a bunch of stupid, poor, frightened people on each side, terrified of the "foreigners," fighting for a patriotic cause that doesn't really exist just so that rich people can take resources and land away from each other. That is all that war is. Fueled by money and stupidity.