This statement of Gene from Chapter 13 is thematic as he has come to the realization that his real enemy is something terribly dark inside himself. Once the War has begun, Gene has an epiphany: his private war parallels the great war.
Because it seemed clear that wars were not made by generations and their special stupidities, but that wars were made instead by something ignorant in the human heart.
Gene's secret evil is the same evil that starts wars. He has waged war with Phineas on three fronts:
- He has suspected Finny of keeping him from making good grades by pulling him away from his studies.
- He has developed feelings of inadequacy as he measures himself against Finny and envies him.
- He has retaliated against Finny by jostling the limb when they are in the tree from which the Secret Suicide Society jumps.
Gene's war is the intrinsic war of man, whether private or magnified on the world scale. In effect, Gene realizes he created his own Maginot Line, overcoming his enemy only after indirectly causing Phineas's death.
In regards to where the quote is located: in Chapter 13. My version places it on page 104, about 15 paragraphs from the end of the book.