What makes Finny decide that the war is real in A Separate Peace?

At first, Finny is in denial of the war's very existence. He comes up with a bizarre conspiracy theory: the so-called war's just a gigantic ruse by a bunch of "fat old men" to keep the kids from crowding them out of their jobs. Apparently, the older generation resent the kids because they remind them of what peace was like in their carefree youth, when they were careless and wild. Finny's denial is reinforced by the school's isolated location in rural New Hampshire. The very idea of conflict just seems so far away from such a perfect, self-contained little world.
It's only when Finny sees what's happened to Leper that he's forced to revise his opinions. Leper was the first boy in Finny's class to enlist in the army. Since returning home from the war, he's experienced significant trauma, suffering terrible hallucinations and a nervous breakdown. The visible damage that Leper, like so many others, has sustained through his experiences of conflict has finally brought home to Finny not just the war's existence but its brutal, unforgiving nature.