FE Fates has am incredible plot: you are the prince of Nohr, but somehow you discover that you are the descendant of the kingdom which is warring with Nohr, Hoshido. At some point of the story you have to chose: fight for Nohr and conquer Hoshido, or deffend Hoshido from the Nohrian offensive(there's a third choice which is go by your own and fight Hoshido and Nohr at the same time).
Personally, I would choose the third choice if what happens in the game has sense, but that's not the case so, if I must choose between Conquest or Birthright, I would choose Birthright. Why you may ask, but I don't have any idea of this choice, I just followed my instinct and that happened.