Jonas, unlike all the other children in the community, understands what war really is. Since the other children have no conception of what death means, they are unable to truly grasp what war is. Jonas knows because he has received memories of war from the Giver and realizes that war means death, and that death means horrible pain for the one dying. It means loneliness, sadness, and grief for those still living.
Because of this, Jonas gets very upset when he finds the other children "playing at war". He gets choked up and tells the others to stop playing that game, that they don't know what they are doing. They respond by saying that it's just a game. Jonas knows better, though, and can't participate, so he leaves.