What had Gene finally come to understand about the cause of war, and how did this contrast with Brinker's view?

Brinker sees war in a very literal way. There is good and there is bad, and they fight somewhere in Europe. Gene sees war as part of the human condition that is not just fought on battlefields in Europe. Gene sees war in his own life and school. Finny is a casualty of war, without ever having been involved in battle. Brinker says this, and Dr. Stanpole reiterates it; but it is not the same kind of war that Finny is really a victim of. He is a victim of a sort of internal war, against yourself, that lashes out against others. Gene hurt Finny because his jealousy and carelessness and his "savage" nature took control of him; Gene didn't hurt himself in trying to get his good nature to win out over the bad, someone innocent was harmed in this struggle. Just as Finny is a victim of Gene's war, so many people turned out to be victims of someone else's war too; perhaps this is why Dr. Stanpole and Brinker see Finny as a war casualty, because his situation is very similar to theirs.