At the beginning of the novel the boys seem to be trying to turn every reference to war into a game. The tree they jump out of is an important example of this since it's main purpose was in training the older boys for the war. Not only do they turn this into a game but they make a club about it. All of this seems to make the war even more distant from them and thus make it easier to cope with.
Gene's sense of peace seems to be disintegrating before everyone else's when he begins to believe that Finny and he are in a battle. When Phineas takes Gene to the beach and later to see Leper jump, Gene sees this as a direct assault on his studying time. In a counter strike Gene makes a direct assault on Finny's sports by knocking him out of the tree.
At this point everyone's sense of peace begins to decompose and the war begins to close in on them.
Next comes the winter session and now the military seems to be eyeing the boys as new recruits, or rather like bodies to consume bullets. While Finny is absent, the war is brought even closer, glorified even, to the other boys when they get visited by the new recruits at the train station. This same day Finny returns and this event seems to temporarily take everyone's mind off of the war.
When Leper leaves though, their sense of peace seems to almost desert them entirely since now one of their own has become apart of the war. The boys, of course, immediately attempt to fortify their position by making up stories of Leper's heroics, now everything good that happens in the war they relate to through Leper. Unfortunately...