Empires were the dominant form of territorial organisation from pretty much the dawn of civilisation right up until the 20th century. Why, at that point in particular, was the form of government which had consistently dominated the world for almost 4000 years replaced by the presence of modern nation-states?
I just want to add to the question. Did the age of empires really end? I mean, wouldn't Europe, Australia, USA, Canada, New Zealand and some other countries fall under "Western empire" from an alien's point of view? Of course, that would be very abstract and with multiple exeptions (like Japan), but the Mongol Empire also was not exactly uniform either. Sorry if this is a stupid question, just want to check whether anyone else more competent in history than me has given this a thought.