Why does Knowles leave the narrative at Devon at this point and discuss the war as Gene later experienced it?
In the beginning of chapter 10, Gene pauses in the story to discuss his own experience as a soldier in the war. His experience wasn't anything dramatic or horrific, so a brief explanation will suffice since Leper's and Finny's experiences with World War II seem to overshadow his by importance (at least to him).
Chapter 10 is a good place to insert his own experience as a soldier because he is about to visit Leper who had just been discharged for a mental breakdown that he had at basic training. Gene, of course, didn't know this before he visited Leper; but, the authorial intrusion (by adult Gene) at this point puts the whole scene with Leper into perspective for the reader to compare together (or what is called juxtaposition). More importantly, though, it is structurally sound to insert Gene's soldiering experience here before the climax of the story which centers around "the truth" about last summer and then Finny's experience and death. It would seem that Gene ends the novel with Finny as an homage to his friend, rather than end it with his "boring" war experience.