Leper actually joins; he has a naive idea that he can be part of ski troops, that do work on skis in the winter slopes. But, basic training is a bit much for him; lack of sleep and all of the pressures causes him to crack. He has a mental breakdown, and is dismissed from the army. He goes home where he stews in his discharge, tormented by the hallucinations that he had had while at basic training. Gene goes to visit him, and is highly disturbed by the more callous, cynical and biting Leper that he discovers there.
Finny is impacted because he wants to join so badly and can't because of his leg. Because of this, he makes up the elaborate tale of the war being a fake, that was conjured up by fat old men in a war room. It is easier to pretend there isn't a war than to keep being denied. He tells Gene, "I'll hate it everywhere if I'm not in this war!" It tears him up that he can't be accepted. So, he invents an alternate reality, but it is one more reason that his broken leg really bothers him.
Gene eventually joins the war, but isn't really impacted by it. He states that he fought his war--with Finny and himself--before the real war ever started. He fought his personal demons way before he had to go to battle. Brinker joined up also, and had a lot of pressure from his father to join and be "where all the action is" up on the front lines, but we don't really know how the war impacts him personally because this happens at the very end of the book.
Eventually, most of the characters are touched by war in some way or another--some dramtically, some barely. Even parts of the Devon campus are turned over to the war effort. War has a way of reshaping everything it touches, and these characters are no different. I hope that those thoughts help; good luck!
A central motif in the novel is fear, never expressed but deeply felt by the boys at Devon beginning in the summer of 1942. As Gene's flashback begins, the Summer Session is underway, as is World War II. Although the war seems far removed from them, it is never far from their thoughts. Jumping from the tree becomes jumping from a sinking troop ship; splashing in the river provides practice for beating the burning oil away as it flames atop the water. Their own war games. Gene notes that the teachers "loosened their grip" on the boys that summer, knowing what lay ahead of them after graduation.
As the school year progressed, the pressure and anxiety of the war that awaited them increased. Each of the four main characters responded to the anxiety in different ways. Leper decided to get a jump on fate. After watching a recruiting film of Nordic ski troops, he enlists before his eighteenth birthday, before he can be drafted. Leper thought he had found a "friendly face" of war, the kind of war he could handle. He was wrong, and it destroyed him.
Brinker's fear was exhibited in several ways. For a while, he dressed like a soldier on campus, an outward denial of his inner terror. Later, he wrote clever and contemptuous poems about the war: "The war is a bore." After witnessing the young faces on the troop train the day the boys picked apples, Brinker composed "The Apple Ode": "Our chore is the core of the war." He spoke often of enlisting, but each plan came to nothing. As he moved closer to the real war, he tried to find a plan to "insulate" himself from danger; his idea of serving in the Coast Guard was one such plan. When his father rejected the idea, fear and bitterness overwhelmed Brinker.
Finny's fear of the war wasn't that he couldn't fight. He simply couldn't stand the idea that the others would go to war, and he would be left behind. Finny thrived on friendship and camaraderie; being left out of anything distressed him unbearably. Finny's joy was found in the unity of the game. Everyone must play and the game must go on. He changed the rules for Leper, at the tree and in blitzball, so that Leper could participate. He fought on both sides of a snowball fight just to keep it going. World War II was happening, and Finny's greatest fear was that he wouldn't be able to play.
The effect of the war on Gene was ironic. He lived with the idea of impending war, as did everyone, but his emotional turmoil was so great in relation to Finny that fear of the war became secondary. His greatest fear was that his betrayal of Finny would be known. At one point, Gene decided to enlist with Brinker (one of Brinker's many enlistment plans) because going to war actually seemed to offer him relief--an escape from his emotional torment.
The war invades Devon in the spring, literally and figuratively, with the arrival of the Sewing Machine squad. War had always been on the way, the Winter Carnival serving as the boys' only time out during that year. By the conclusion of the novel, World War II became real for the boys at Devon, and innocence had been lost.