Why has no person / group ever taken over the world?

Communication as a constraint (along with transportation as a constraint [LW · GW]), strikes me as important, but it seems like this pushes the question to "Why didn't anyone figure out how to control something that's more than a couple weeks away by courier?"

I suspect that, as Gwern suggests, making copies of oneself is sufficient to solve this, at least for a major outlier like Napoleon. So maybe another version of the answer is something like "Nobody solved the principle-agent problem well enough to get by on communication slower than a couple weeks". But it still isn't clear to me why that's the characteristic time scale? (I don't actually know what the time scale is, by the way, I just did five minutes of Googling to find estimates for courier time across the Mongol and Roman Empires)

"There's also a dynamic commonly seen in 3+-player strategy games where, if there's one superpower close to reaching the power level of everyone else combined, then everyone-else will ally to bring them down, maintaining a multipolar balance of power."

This does seem to happen at time (think Peloponnese war or Westphalien peace Europe), but local hegemon who have eliminated all their competitors also existed (Roman empire, or post WW2 USA).

Your first point, however, is spot on.

If you define "the world" as everything with in reasonable comunication distance (given the technology of the time) then there are several examples of some group talking over the world. E.g. every time China was unified.

I'm pretty sure that's the whole purpose of having province governors and sub-kingdoms, and various systems in place to ensure loyalty. Every empire in history did this, to my knowledge. The threat of an imperial army showing up on your doorstep if you fail to comply has historically been sufficient to ensure loyalty, at least while the empire is strong.

orders take a long time to travel so you can't meaningfully control things that are far away

Wouldn't that just slow down things?

Also, a counter-example: The Romans managed their empire (diameter ~4000km) quite well in that regard for a few hundred years. To get to world-scale you'd only need to speed up communication by a factor of 10.

It's funny to consider that briefly before the electric telegraph, the French developed a system based on people stationed in chains of towers with telescopes signalling to each other. This strikes me as an idea behind its time.

EDIT: On further research (meaning, reading Wikipedia articles) it seems optical and hydraulic telegraphs were attempted in various places, including 4th century Greece and 9th century Turkey (Byzantium) so it seems this is not so behind its time.

if there's one superpower close to reaching the power level of everyone else combined, then everyone-else will ally to bring them down, maintaining a multipolar balance of power.

I hope they don't use nukes when they do that because that way, everyone loses.

I don't think that we've even come close yet. We've had some windows during which someone could have (weakly!) imposed their will on anyone in the world, but not on everyone.

Why 2010? In my opinion, we are still in this era, for better or worse.