A thesis might be made posing Gene's internal conflict as a metaphorical battle between the impulse to remain a child (and follow Finny) and the impulse to plunge into adulthood (following Brinker).
The opposing forces in this metaphor can be examined in an essay looking at Finny's relationship to innocence and childhood, as well as his false denial of the reality of the war in Europe. Brinker's persona and his eagerness to act responsibly can also be examined.
Early in the novel, when Finny is the leader of the boys, Gene declares:
"I think we reminded them of what peace was like, we boys of sixteen. We registered with no draft board, we had taken no physical examinations."
This statement and similar ideas are repeatedly attached to Finny, while an opposite set of attributes becomes associated with Brinker.
The war, as it exists for Gene, is represented by these two figures.
And, for Gene, Finny, Brinker and the rest of the boys at Devon, the war is a personal war. The peace that Gene finally realizes is therefore also a personal peace. Focusing on Gene's internal conflict works on at least two levels for an essay.
His internal conflict is a war, in and of itself. Dueling impulses battle for dominance as Gene decides if he wants to follow Finny's path and hold himself off from the war and from adulthood, or if he wants to rush into responsibility and enlist as Brinker recommends.
Gene's internal conflict also has a metaphorical relationship to the war. In a way, we might argue that Gene's turmoil and internal struggles function as a type of psychlogical displacement.
The war in Europe is translated to Gene's speculations and symbolically is represented in his internal conflict. Seen in this way, the war in Gene to choose between Finny and Brinker is a direct metaphor for the inevitable future physical life-and-death battle that awaits him in Europe. Only here, at Devon, the war takes the shape of a conflict between childhood and adulthood.
This internal conflict would not carry the poignancy it does were there no war waiting to pull the boys of Devon in. Under different circumstances, a boy like Gene would have more time to come into his own sense of self.
In the end, Gene realizes that the war going on inside him is unnecessary. He does not have to choose to follow either Finny or Brinker. He can choose his own path. He had been wrong about his options. What he thought he knew had led him to turmoil. His epiphany, in the end, brings him the peace of a new and significant piece of information. He has the option to be himself. Until Finny dies, Gene remains unaware of this possibility.
The conflict in his heart is given analogy in his thoughts on the war, when he thinks:
"that wars were not made by generations and their special stupidities, but that wars were made instead by something ignorant in the human heart."