The boys first talk about the war in childish terms. They name games after war jargon. They rationalize the jumping from the tree as some sort of training. Finny wears crazy clothes as some sort of war tribute. The war gradually creeps into their lives as a reality. Recruitment officers come to the school and paint a false picture of safety that only Leper falls for. They pick apples to make up for the lost workforce without ever pondering the realities those young boys are facing. They clear railroads so that new recruits can get out and face the war, thus getting them to danger that much faster. The one moment that makes the war most real is when Gene realizes that Finny is out of the war because of his actions.
The biggest factor was the enlistment of Leper, the boy least likely to become involved in the war. Suddenly, the boys are adament about assigning all great allied victories to Leper. Most likely, this is their way of coping with the fact that they, like Leper, will soon be a part of the action. Gene describes this "separate peace" they have at Devon on several occasions. When Leper enlists, they can no longer avoid their imminent futurers. This single act draws them, involuntarily, into the actions of the war.