At the exposition, thats the asnwer
C. Germany's attack on Russia was similar to betraying all the rules of civilization.
Russia's intention was clearly aimed at making peace with Germany, otherwise they would not have signed the peace treaty. The fact that Germany then turned around and broke that treaty because of how horrible their leaders were, shows that they were not following established rules of civilization.
I believe that an unhappy ending can do that.
When a story has a happy ending, then you finish it and you feel satisfied and joyful about how the story ended. On the other hand, if the story ends in an unhappy manner, you start thinking about the resolutions and imagining what could lead to such an ending and how it could be fixed.