20 days is not a very long time, and Europe is a large continent, so don't spend too much just getting from here to there. For that reason I would stick to the northwestern corner of the continent if I were you, there's plenty to see there and you will then spend your time seeing places rather than sitting on trains. I'd probably check out Berlin and Cologne in Germany, one or two Belgian cities (Brussels, Antwerp, Ghent or Bruges), maybe Paris (although not one of my personal favourites, but nevertheless one of those 'see once in your lifetime' places).
That's already quite a lot, giving you only a few days in each.
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.