In A Separate Peace, Finny loves to do what he can't or isn't supposed to do. He wears a tie for a belt then makes up a preposterous reason for doing so, and manages to talk and rationalize his way out of it. He breaks the swimming record because the record exists and no one else has, and because he isn't a swimmer so he's not supposed to be able to break it.
Finny shows no interest in going to war before his "accident," but once he's crippled and can't go, then he wants to. Joining the military becomes an obsession for him. Why? Because he can't.
He perpetuates the myth that there is no war, the same way he got out of using a tie for a belt: creating preposterous rationalizations to prove the war is a fake, and repeating them and adding to them whenever the subject comes up.
Only after he has allowed himself the destruction of his illusions, only after he admits to himself that Gene purposely jounced him out of the tree, does Finny admit the truth about his failed attempts to enlist.
After his accident Finny starts telling people that war is not real. He claims that people have made it up. Later in the story after Gene has been placed on trial in front of Finny and Finny finally hears the truth, he falls down the stairs. While in the hospital Gene goes to visit Finny.
Finny explains to Gene his rational about war not being real. It is no longer real for him. Because of his leg he had been turned down by all of the military branches. It had been extremely important to Finny to serve in the military, so when he keeps being rejected, was ceases to be real for him because he will never be a part of it.