What is a Maginot Line? Why does Gene mention it in the end?

This comes as Gene's final reflection on his relationship
with Finny and his time at Devon. His stream-of-consciousness shows his internal
censorship, and his attempt to come to terms with his
life.

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.

The
Maginot Line was a defense structure that France built along its border with Germany and
Italy during WWII, which included concrete walls, machine gun posts, artillery
casements, and tank obstacles. The idea was to deter a direct assault on
France, thereby giving them enough time to muster their army. This approach
worked...kind of. Germany didn't attack France directly, but did attack Belgium, skirt
the Maginot Line, and invade the country anyway.

In the
novel, we can see the connection to Gene's memories. Essentially, he is admitting that
he imagined Finny as an enemy, one who never actually attacked Gene; in fact, Finny
never considered Gene anything other than his best friend, and certainly would never
hurt him in any way purposely. So, for Gene, his defenses against Finny
were all for nothing, much like the Maginot line. Gene also imagines that
all the boys at Devon (except Finny) constructed these defenses against each other, but
all for nothing. His last thought is that perhaps the true enemy was inside each of them
all along.