Realise that Egypt, Greece, Macedonia, Rome, Persia, and China each spanned a thousand miles or more, the most effective transportation was over water, either along rivers or across seas or oceans, that ocean travel was impossible for much the year (Roman vessels were restricted to port from November through May, this lasted until the 1300s in Europe), and the minimum time for a message to traverse a thousand miles was easily ten days, if not months.

You needed autonomous lieutenants in place who could be given general orders (much like goal-seeking AI, now that I think about it), be trusted to be only modestly corrupt, not collude with enemies or others against the centre (a frequent problem), and to truthfully report what they'd experienced, in words -- writing existed, but not photography, video, audio, etc. Testimony, that is, someone's testement or attestation of fact, was all you had, though multiple testimonies could be compared against one another.

I find it interesting that every major empirical power had some intrinsic religion, probably serving as a moral check and guidance, a role that's often underappreciated today. Also that other than a set of strictures, the religions themselves often had little in common with one another: polytheistic vs. monotheistic, theistic vs. meditative, commandments vs. ancestor worship or reverence.

It's a topic on which I'm almost wholly ignorant, but find fascinating.

I think unappreciated problem with modern communications tools is that by default, they enable and encourage micromanagement.