In A Separate Peace, why does the author leave the narrative and discuss the war as Gene later experienced it?

There may be several reasons for the author leaving the
narrative to discuss the war as Gene later experienced it.  First of all, Knowles
reminds his reader that the story is told in retrospect.  In the first chapter we learn
that Gene is returning to Devon as an adult, trying to make sense of what happened
there. So reference to future events would not be
inappropriate.

Secondly, the mention of war serves to
connect World War II with the events that happened at Devon.  We see how flawed
individuals are, how easily we are tempted to turn on one another as Gene's jealousy
toward Finny caused him to jounce the limb of the tree.  But we also see see that we all
fight our own private wars, needlessly, against enemies that are not truly our enemies.
 As Gene confesses,

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I never killed anybody and I never developed an
intense level of hatred for the enemy.  Because my war ended before I ever put on a
uniform; I was on active duty all my time at school; I killed my enemy
there.

Gene's true enemy was
not the Japanese or the Germans, or even Finny.  His true enemy was himself and his own
jealously and insecurity.  This realization causes him to obtain his "separate peace."
 In this way, we see that Finny, even though he died, had a lasting effect on Gene and
that the events at Devon serve as microcosm for the bigger war that was raging on the
outside.  The enemy may not be who we perceive it to be.  It may be, as Gene discovered,
ourselves.