I'm currently doing some research for my first book which involves a planet with an empire that has had its roots in Antiquity. It's basically a country that formed about 2200 years ago and has started to take over the world 1500 years ago, forming a global empire around 200 years ago.
What would a global empire, grown from a country that was born during antiquity and took over every other nation by a combination of force and diplomacy look and feel like? I understand that the Romans kept their empire together against internal struggles by assimilating the different cultures. Would that work on a global scale? Or should it go the entirely opposite way, with an empire only in name and extreme independence for every conquered state?
I mainly would like to know 3 things:
how can such an empire stay in power without losing grip on the distant parts?
How do the logistics of such an empire work? Would their world economy be better or worse than ours, assuming similar tech level?
How would the empire keep the military at a functional level? there are no other countries to wage war against, so I think there would be a pretty high likelihood of the armed forces just not staying at a proper level.