Should the USA refrain from playing policeman to the world, and just let them all kill each other?

No, of course not. The US will eventually spread its resources so thin it will not be able to bounce back.

( Je suis Charlie, Paris & San Bernardino / I'm Charlie, Paris & San Bernardino) America is not ' playing ' the world policeman this time, since it is not alone! There are plenty of other countries amid the coalition : France, Canada, Belgium, Australia, more recently Germany and UK.. etc.. Best Cat.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.