According to Gene, "wars [are] made...by something ignorant in the human heart." What line from Chapter Twelve does this quote seem to echo?

I'm not exactly sure which quote you are looking for. Finny was the only one who didn't show ignorance and Gene echoes this when he talks about Finny being in a war, "you'd get confused and borrow one of their uniforms, and you'd lend them one of yours. Sure, that's just what would happen. You'd get things so scrambled up nobody would know who to fight any more." (chapter 12)