Marginalium

Empires as a function of transport technology:

This brings a new light to the two transportation assets Romans were famous for: the Mare Nostrum (Mediterranean) and the roads. The sea allowed for fast travel across the Mediterranean, uniting it—but preventing Rome from going much beyond it. The roads were necessary for Rome to move past the coasts and control the land.

While London, the upper Nile, the Levant, and even the Black Sea could be reached in less than a month, the lands beyond the Rhine river, today’s Germany, couldn’t.

And this is in a world where they had no military or economic equal. As neighboring areas grew stronger, one month of distance was too remote to hold. Rome abandoned Britain, Germanic tribes invaded the European side, the Sasanid Empire on the Asian side, and the half of the empire farthest from Rome split.