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.
With dramatic irony the Second World War goes from being a distant something to be nonchalant about to a foreboding presence in the lives of the boys at Devon. In Chapter Seven Gene narrates, "In addition to classes and sports and clubs, there was the war. Brinker Hadley could compose his Shortest War Poem Ever Written: 'The War/is a bore.'" Later at apple-picking time, Brinker is inspired to write "Our chore/Is the core/of the war." And, Finny, who takes nothing seriously, jocularly throws out words like "Central Europe," and news stories that no one else has seen; later, Finny creates the game of Blitzball, named after the sudden, overwhelming and deadly Nazi attacks known as the Blitzkrieg.
After poor Leper has the breakdown and Section Eight discharge, Brinker, who once scoffed at the war tells his father that he is enlisting as does Gene so that he can avoid the infantry. Gene later reflects that "wars were not made by generations and their special stupidities, but...were made instead by something ignorant in the human heart." No wonder that the older Gene returns to Devon to seek "A Separate Peace."