How is the war present in the book A Separate Peace?

In the book, war will impact all of them in one way or another.  A lot of the faculty fought in the first world war, or knew people that did, and probably lost friends and family there.  For the students, a lot of them will end up fighting in the second world war, as is evidenced as the book progesses.  Not only this, but the students see trains full of soldiers off to war, which prompts a lot of them to go join. They help the war effort at the train station.  Also, near the end, parts of their school are vacated and donated to the war effort for storage and training.  It changes everything that they know, their entire world.

For specific characters, Finny is impacted by the war because they won't accept him.  He desparately wants to join the war, but because of his leg, can't get in.  From this sprouts a lot of his schemes, plots, and activities-a way of staying busy to stay in denial about it all.  Then for Leper, we see the war be an actual catalyst to his mental breakdown, and eventual betrayal of Gene, something that was to harsh for the "old" Leper to have done.  It shapes and changes his entire personality.

In one way or another, every character in the book is impacted by the war; it is an underyling force throughout the book, always there, hovering, waiting, and it changes them all.