In The Giver, how did Jonas react to the children’s war game?
In chapter 15, the Giver shares a difficult memory of war with Jonas. Jonas recalls being injured on a battlefield in a confusing, foul-smelling location. After witnessing a horse get shot, Jonas turns to his right and sees a young boy in a gray uniform begging for water. Jonas, who has been shot in the arm, opens a canteen and reaches out to give the injured soldier a drink of water before he dies. This traumatic memory gives Jonas insight into the nature of war. In chapter 17, Jonas witnesses Asher, Tanya, Fiona, and some other children playing a simulated game of war, in which the children pretend to shoot each other. Jonas, who has experienced the overwhelming sadness of war, remembers seeing the boy die on the battlefield and pleads with the children not to play the game anymore. Jonas refuses to play the game and does not move when he is "shot." Unfortunately, Jonas cannot express his thoughts and share his experiences with the children, so he quietly walks away. Essentially, Jonas's childhood carefree nature has been lost and he is filled with an overwhelming sense of sadness.
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.
You can find the answer to this towards the end of Chapter 17.
Seeing his friends Fiona and Asher and the other kids playing the war game totally freaks Jonas out. It makes him feel terrible, so much so that he just about starts crying.
The reason for this, of course, is that he (unlike any of the other kids) has actually experienced what war feels like. He has lived the experience of the soldiers who were wounded and dying in the war in the memory the Giver gave him. Because of that, he cannot enjoy the game the kids are playing.