Slow communications and travel made overly-large empires ungovernable. I mean, there's something almost deterministic about the fragmenting of the Mongol Empire. But what if someone had invented the bicycle in 200 BCE?
A good horse and rider team can cover 150km in a day. A fit ultracyclist can cover 750km, or 5000km in a week. Even allowing for lower quality materials, it doesn't seem unreasonable to suppose that an ancient cyclist could have covered at least half that distance.
If it had been possible for a Persian army to travel from Susa to Sardis inside of a fortnight, or for a Babylonian force to reach Gaul in a couple of months, surely history would have been different? I mean, apart from ancient warriors looking more ridiculuous than they actually did. Centralised bureaucracies could have exerted direct control over larger areas. Mobilisation of forces could have taken place more swiftly. Julius Caesar could have called his memoirs "the Tour de France". "All Gaul is divided into a number of stages," he would have written...
My point is, the "for want of a nail" poem should have been called "for want of a bicycle". Also, historical determinists can go jump in a lake (provided they have a bike and said lake is within 750km).
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