Why are some nations rich, while others stay poor?

Hello, I'm writing a paper at a High school level and I'm interested in what you guys think about this issue?

Currently I'm comparing Germany and Moldavia in order to keep this project to a reasonable length. Even though my main focus is on getting this assignment done I were able to pick the subject myself and I'm genuinely interested in digging deep for some sort of answer, although I understand it may be a very dynamic one.

Thanks for any help I may receive! :)