What did Gene actually mean when he said, "my war ended before i ever put on a uniform; I was on active duty all my time at school; I killed my enemy there"?

Gene says this towards the end of the novel. He reflects that his school experience at Devon was very much like a war. He muses that like war, the world is an essentially hostile place. People experience pain both emotionally and physically just by experiencing life. Gene feels he had been through a battle at Devon. Gene's enemy is not elaborated on but we can surmise it has something to do with his relationship with Finny and the intense teen angst that surrounded it. Gene feels that his casualty of war was the loss of his innocence. He will enter his adult world already battle scarred and hardened.