There were two major things that led to Gene and Finny finding their "separate peace." Obviously, their surroundings played a huge role. They were experiencing new things with the war coming into their lives. They didn't know how to deal with it. So they created a separate peace. The other thing was the fact that Gene and Finny use the fact of having inner enemies to help find their separate peace. Gene didn't really find his separate peace until he returned to the Devon School many years later.
The first discovery was World War II. It symbolized conflict and enmity, or hatred. Gene felt that hatred was a natural part of adulthood. Most people fight in a private war against a private enemy in the novel. Even throughout peacetime, the boys are defending themselves against their individual enemy. Finny is the only boy that doesn't have this problem. Finny refused to have hatred, or an enemy. He denied that the war existed because he didn't have enmity. Finny found his separate peace within his denial. Gene later told him that he could never be a soldier because he didn't understand what having an enemy was all about. The war didn't really enter their lives until after Finny's fall. This means that the war will stay with them until Finny's death.
WWII affected the characters in many ways. Gene saw that football was like the battlefield and he saw tennis balls as bullets. Gene thought of the snow as an army forced upon the Devon School (85). The war caused the boys to feel unsteady. It was disturbing their earlier lives but not letting them move on to the next chapter.
In addition, Gene and Finny shared the same dreams of the Olympics still going on as planned, and that there is no war. This is where Gene and Finny find a separate peace. They felt safe and secure when they knew that the Olympics would be in 1944 and that there was no war. They didn't have to worry about dying. They didn't have to worry about their best friend.