At the end of A Separate Peace Gene (John Knowles) compares life at school to World War II. He says, "I was on active duty all my time at school. I killed my enemy there." Does he mean the enemy was the other boys or something within them?
In 1942, Gene and Finny (Phineas) are roommates at boarding school. Gene is adequately athletic but Phineas is the school's star athlete. Gene harbors some secret feeling of suspicion about Finny's motives in friendship, thinking that perhaps on occasion Finny tries to sabotage Gene's studies. Gene suspects Finny to be as envious of Gene's scholastic abilities as Gene is envious of Finny's athletic prowess. Gene feels inferior to Finny when evidence indicates that Finny respects Gene's studiousness.
Out of these sense of inferiority and its attendant guilt, Gene agrees to go to the river to watch Leper jump from a tree branch into the river. It is here that Finny suggest a double jump to Gene and then Gene for an undisclosed motive--maybe just a childish impulse--jostles the tree branch to cause Finny to lose balance and fall--into the river. But Finny falls to shore and shatters his leg.
At the end of the story, when Finny has died and Gene is going off to war, having no feelings of hatred, and he says that he killed his enemy at school, he also says that he believes the boys at school created something of an enemy between themselves that was built out of their fear. Once Finny died, Gene had to face his guilt and loss and remorse, and since he had lost the symbol of the enemy (Finny) but the enemy remained with him, he concluded that the enemy was the fear the boys felt. Once Gene recognized the enemy was never in the superior qualities or abilities of Finny, he could say that his war had ended before he "ever put on a uniform."