And now for something completely unimportant. I'm still thinking about this recent article on how access to maritime trade explains a lot about ancient history 1/ voxeu.org/article/trade-… 
And I have some amateur thoughts I'm sure real historians will consider silly. But whatever. Given the huge advantage of boats over roads in pre-industrial society, the Mediterranean made a lot of sense as an economic and political unit 2/
That is, the Roman Empire was in a way the kind of thing you'd expect to see. In fact, however, the Mediterranean was never again the center of an empire. For a time the Ummayads controlled about 3/4 of the coast, but they weren't really centered on the sea 3/
And it didn't last; instead, you got a sea surrounded by land-centered states. What changed? Why didn't something like Rome's empire reappear? I don't think hypotheses like "the Arabs weren't sailors" work, because such things can change over time given incentives 4/
So, two technological hypotheses. First, the heavy plow, introduced in the early Dark Ages. 5/ eh.net/eha/wp-content…
This brought a lot of heavy clay soil under cultivation: regions that were sparsely populated by barbarians under Rome became major pop centers. Maybe shifted center of gravity inland? 6/
Second, the stirrup. Made cavalry much more effective. Since horses are harder to put on ships than infantry, did this also reduce the scope for maritime empires, at least maritime empires that extended their reach inland? 7/ scientificamerican.com/article/the-st…
I'm well aware that I have no idea what I'm talking about. But the long-term division of the Mediterranean, which seems so natural a unit -- and was in the ancient world -- into Christendom to the north and Islam to the south seems to demand an explanation 8/
We now return to our regularly scheduled anxiety about the crisis of the American republic 9/
