Gene falls into the “dirty" river. Gene is at the point where he is reveling in his guilt, and in the negative thoughts he has been having. Now that Phineas is gone, the joy is gone from Gene’s life, so it’s only fitting that he gets into a fight AND gets thrown into the filthy river as punishment for what he has done.
Throughout the chapter, Gene is terrified of being found out, even though it is highly unlikely that anyone truly suspects anything.
Leper provides contrast to the other boys at the beginning of this chapter. Leper seems to be actively involved in living in the moment, while the rest of the boys are so anxious to move forward. He enjoys “stopping to smell the roses," so to speak, while everyone else is training to be a soldier. He misses a lot of “important news" because he’s too busy enjoying nature and the world around him. The other boys don’t take too kindly to his attitude. Essentially, they mock him for not being patriotic enough. They claim that he is an enemy of the state just because he wants to get his education before he enlists. Nowadays, soldiers have to finish high school before they can enlist; or at least they have to pass the GRE. But back then, to hold back was to show cowardice.
When they actually see soldiers going off on a train toward the war effort, the Devon students realize that all they’ve been doing up until that point has been futile. They can pretend they’re training, but the men in uniform are the ones who are really doing something for the war. It makes them feel like they’re just little boys again, surrounded by “real men" who are ready to fight. It puts a face on the war for them.
Gene is driven to enlist by the incident at the train yard, as well as by Brinker’s confession that he is ready to enlist. Gene wants to join the war, where the dangers will be real and not just in his head. He wants to make a solid decision to do something, rather than waiting to be told (enlisting now vs. being drafted later). He sees the war as an inevitable part of his future, and he would rather face it head-on than wait for it to come to him. Gene wants to defend the freedom he enjoyed that summer at Devon. He wants to protect his country and the freedom he has felt there.
Finny’s return changes Gene’s persona. During this chapter, he has made his own decisions. He chose to punch Quackenbush; he chose to help with the snow shoveling; he chose to enlist. Now that Finny is back, he realizes that he will go back to deferring to Finny for all of his choices.