Matt Yglesias offers a good review of this excellent movie, which chronicles the early life of Genghis Khan, or one vision thereof. There are at least two increasing returns to scale mechanisms in this movie. First, leadership is focal, which tends to bind groups together and make concentrated rule possible. Winning battles makes you focal and winning larger battles makes you focal across larger groups. Second, if you walk or ride alone in the countryside, you will be snatched or plundered. That causes people to live in settlements and also larger cities. Put those mechanisms together, solve for equilibrium, and eventually one guy rules a very large kingdom and you get some semblance of free trade. Sooner or later, that is. The movie brings you only part of the way there and I believe a sequel is in the works.