Three themes

The three main themes in the novel, A Separate Peace, include war, competition, and friendship.

Friendship:

"It's you, pal," Finny said to me at last, "just you and me." He and I started back across the fields, preceding the others like two seigneurs.

We were the best of friends at that moment.

War:

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, which included jumping from this tree.

Competition:

The Devon faculty had never before experienced a student who combined a calm ignorance of the rules with a winning urge to be good, who seemed to love the school truly and deeply, and never more than when he was breaking the regulations […]. The faculty threw up its hands over Phineas, and so loosened its grip on all of us.