Although using nonlocal is the direct answer to your question, I would encourage you to consider using a callable class here. This avoids re-defining a function every time cap_sentence is called, and it handles state a more obvious way (to me, anyway). I've taken the liberty of adding a return statement at the end, so that you don't get a string of None values.
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.