Since the first book was written in 2005, it is quite safe to presume the story begins in 2005. By the 5th book, the year is also presumably 2009. There are 5 books because each book takes about 6 months to write so the author Rick Riordan probably decided to put one in the middle (the 3rd book: The Titan's Curse).
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.