Thanks, I already understood what Birthright was like, I just figured that some of that functionality is given by Dynasties & Demagogues.
I think a better way of looking at it is to say that Dynasties & Demagogues (DaD) gives you the ability to roleplay political situations, whilst Fields of Blood/Birthright let you wargame/metagame similiar sorts of situations.
DaD will allow you to roleplay the leader of a nation, but it doesn't help you or the GM know how much tax will be raised this year, or whether crops were good or bad, or what the military maneuvering of your neighbours really means to your realm. The GM can wing a lot of that stuff, but FoB and Birthright give you the ability to put numbers on it. Birthright goes farther than FoB, because it allows a group of PCs to each be their own center of power within the same realm - DaD allows that as well, just on a roleplaying level.
By mixing FoB and DaD you could (but would you?) have players roleplay out the interaction between different power groups, and then resolve the regent/realm actions appropriately.