Explain the sentence structure, especially in the last three paragraphs in A Separate Peace and how it adds to our understanding of the novel.John...

Gene enlists in the navy, but he feels no patiotism, feeling that war is the result of "something ignorant in the human heart."  This belief causes Gene to be introspective, as the structure of the last three paragraphs evidence.

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.

The use of independent clauses simply joined by semicolons lines the three statements evenly.  Gene lays his confession out tersely with no apologies; it is very effective in its candidness. He condemns himself point-by-point.  While his hat was a confusion of love and hatred, Gene finally achieves order in his confession.

Only Phineas never was afraid, only Phineas never hated anyone.  Other people experienced this fearful shock somehwere, this sigting of the enemy and so began an obsessive labor of defense, began to parry the menace they saw facing them by developing a particular frame of mind.  "You see," their behavior toward everything and everyone proclaimed, "I am a humble ant, I am nothing, I am not worthy of this menace," or else....

Gene's second paragraph is more a stream of consciousness as he confesses his and others' unworthiness.  The fear of Gene and the others made them less that Phineas.  They could not face their inadequancies.  Gene's search has been for exact and individual standards.

All of them, all except Phineas, constructed at infinite cost to themselves these Maginot Lines aainst this enemy they thought they saw across the frontier, this enemy who never attacked that way--if he ever attacked at all; if he was indeed the enemy.

Again affirming the superiority of Phineas, Gene reflects that he and the others tried to protect themselves from their envy and fear by creating a psychological wall between themselves and their "enemy."The parallelism of this long sentence is the final realization of Gene in a single thought.  "if he was indeed the enemy" tells that Gene knows the enemy is oneself who has that "something ignorant in his heart."