Yeah thanks Rodan. Good points.

I'd probably say the first of those things is to do with the whole 'momentum of empire building' thing. Once you have an army, you have two choices: leave it doing nothing (in which case it gets either lazy and bad at fighting, or bored and keen to find some violent outlet, possibly involving your royal palace) or send it elsewhere. In other words, once you've got the army, if you don't use it, you lose it (and everything else). That's the Roman story. Just as an aside, when reading about empires in history, you tend to find their growth is always linked with some 'great leader' - it's easy to forget that the soldiers who are doing the fighting are not just parts of a machine, they're real people; they have their own fears, hopes, wants, etc. Often it is that reality, rather than some ruler far away, that effects historical events.

I'm unsure about the environmental factor though. There doesn't sound like anything wrong with the idea, but I don't know of any empires that have gotten started due to some kind of environmental event. I can think of a few that came to an end - the Minoans, Mayans and Greenland Vikings being prime examples - but none that started. That's actually not entirely true. The Mongols, arguably, started rolling across Asia because their grazing land was shrinking. But it's hard to be sure, since they also had a very aggressive culture, and Ol' Genghis unified the tribes for self-defence reasons, not to gain more land. At least, as far as I know. Also, I can easily imagine that empire's might start in the future because of environmental causes.

Anyway, food for thought. I thought tomorrow I'd try to write about the origins of cities.