They trained their armies very well so they would be ready for another war like WWII if one happened again.
Earth can be considered as a closed system, since it only receives sunlight (energy), while the overall mass stays constant, without (almost) any exchange from space. Another example of a closed system is a saucepan or frying pan, on a stove, when its lid is closed.
This question is really opinion based, but I would take it personal when things go wrong, since I start to think what I didn't or did do that made things go wrong.
