How are the themes of warfare, search for identity, and jealousy used in A Separate Peace by John Knowles?

When the action of the book starts, Gene is a high school student faced with an identity crisis, and he is filled with fear. He writes of that time, "We were in shaky transit that summer from the groveling status of Lower Middlers to the near-respectability of Upper Middlers." Even Gene's status in the school, caught between the youngest kids and the oldest, is unclear. Gene, an intellectual, is unsure of his identity, and he befriends Finny, a popular boy, in part to cover up for his own insecurities and to feel less afraid. 
Part of Gene's fear has to do with the war, as the book's action starts in 1942, during World War II. He says:




"The class above, seniors, draft-bait, practically soldiers, rushed ahead of us toward the war. They were caught up in accelerated courses and first-aid programs and a physical hardening regimen."

While Gene and his classmates are still "numbly reading Virgil," students just a bit older than them are preparing to enter the war and to leap into the unknown, worsening Gene's fears. 



While Gene admires Finny, he is also jealous of his friend's ability to get away with behavior none of the other boys would dare exhibit. For example, when Finny wears a bright pink shirt, the teachers only find it funny, not punishable. Gene thinks:

"I was beginning to see that Phineas could get away with anything. I couldn’t help envying him that a little, which was perfectly normal. There was no harm in envying even your best friend a little."

Gene's insecurity and confused identity, worsened by the coming war, makes him more and more jealous of Finny until he decides to deliberately shake the branch of the tree on which Finny is climbing.