The last line from A Separate Peace:
. . . 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.
After World War I and building up to World War II, France built the Maginot line, a series of military fortifications, posts and defenses along the French-German border.
Only Phineas was innocent enough to look at the world without the necessity of declaring an enemy. There is a symbolic reference to Gene causing Phineas' fall here, but Gene was jealous of Phineas' innocence; he didn't really hate Phineas. However, Gene was jealous of Phineas' ability to look at the world without needing an enemy. This is a lesson Gene would learn, perhaps only later in life, in a larger sense but also in terms of how he framed Phineas as both friend and enemy. One question remains: Was Gene just jealous and/or did he also need an enemy, much like the adults do as they gear up for the war with unnecessary fanaticism?
Relative to the war, it's clear that Brinker and Gene are not as enthusiastic as their fathers. In fact, Brinker blames the upcoming war on their fathers' generation. The last line underscores the naive but also simplistically wise and innocent way Phineas looked at the world. Referring to the last quote, everyone else (even if not so crazy about the glory of war) does tend to built up a defense (Maginot line) as if there is always some enemy out there. Phineas never felt this way. The last line, and the entire novel, pose the question about whether "needing an enemy" is part of human nature or if it is a cultural tradition passed on from our forefathers. If the innocence of Phineas is any indication, then it is the latter and therefore, something we can overcome. Phineas did not need an enemy and when he comes back to Devon after his accident, he restores peace.