World War II influenced the boys in A Separate Peace, by John Knowles, by making them grow and mature more quickly than they would have had there not been a war. The war made some boys stronger and readier for whatever life would bring, while in others it disabled them to the point that they could not handle the demands of life.
The maturing influence of the war on Finny was a considerable one, even though it did not seem to the other boys that he was growing up at all. Gene's jealousy leads him to the point where he has to destroy Finny's greatest asset, his skill in sports, just so that he does not have to be the "popular guy's friend. Gene knocks Finny off the tree limb and he breaks his leg. Everyone at Devon except for Finny suspects that Gene caused Finny to fall off the branch, not Finny's loss of balance.
Finny's outlook on the whole situation is very grown up. He did not blame anyone but himself, even though it was not his fault at all. Finny seems as though he will never grow up because he is so immature, with his silly denial of the war's existence, and his habit of always coming up with strange things to do just for fun. Inside he is suffering with the anger and hurt of being excluded from the one thing that he wants to do most, fight in the war. This is an excellent example of how the war suddenly made the boys grow up into men. They had to face adulthood, and in order to do that, they had to become adults.
Another boy in the story who was made to grow up by the war was Leper. When he sees the movies about the ski troops, he thinks that it looks fun, so he surprises everyone by enlisting. Leper did not quite know what he was getting into when he enlisted. He thought that it looked like a fun ski trip; he could serve his country and ski around the world at the same time. Most average teenagers are not faced with the pressure of a war and shortages of everything, and so Leper had to grow in order to