Describe the structure of the sentences in the last three paragraphs of A Separate Peace. Why are they effective ?

These paragraphs are constructed of very long complex sentences-in many cases, run-on sentences. This connects to the overall viewpoint of the novel, that of Gene Forrester. The novel is narrated in a controlled stream-of-consciousness mode, in which we are inside Gene's head, yet he is censoring some details the audience and himself. The long, rather rambling sentences reflect his own incoherency and attempt to deal with  his actions.

Only Phineas never was afraid, only Phineas never hated anyone….All of them, all except
Phineas, constructed at infinite cost to themselves these Maginot Lines against this enemy they thought they saw across the frontier, this enemy who never attacked that way – if ever attacked at all; if he was indeed the enemy.

You can see in this quote how Gene's mind wanders from thought to thought, attempting to make sense of what's occurred at Devon.

Another striking stylistic trait of these sentences is the use of parallelism. Parallelism is a balance of two or more similar words, phrases, or clauses. Essentially, it is a parallel structure of any of these elements. For example:

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.

Here, the repetition of "I never" not only compares the idea of killing with the concept of an enemy, it also makes it sound like Gene is trying to convince his audience of what he's saying. Perhaps too, he is trying to convince himself. It's almost as though as an audience, we are listening to Gene think through his argument, and reason out why he did what he did. Finally, the extensive allusion in the last paragraph connects Finny & Gene's relationship to WWII in a metaphorical way, underscoring the impact the war had on all of them.