Finny refused to accept the war as being an actual event that really was occurring for most of his time in The Separate Peace. In his opinion, the war was being presented by the "fat old men" who ran the country and needed a way to control business and the youth of the nation.
The fat old men who don't want us crowding them out of their jobs. They've made it all up. There isn't any real food shortage, for instance. The men have all the best steaks delivered to their clubs now.
The shock that forces Finny to accept the reality of the war comes when Gene reports on Leper's mental breakdown. The transformation of Leper from a quiet, peaceful person in touch with the world to a frightened, crying rebel willing to go Absent Without Leave rather than continue in his assignment is so radical as to convince Finny that there must have been real, awful circumstances impacting Leper.
"Sure. There isn't any war." It was one of the few ironic remarks Phineas ever made, and with it he quietly brought to a close all his special invention which had carried us through the winter. Now the facts were re-established