"Yet when all is said and done, neither comets, nor strategy, nor even loose morals by themselves drove Western social development down in the sixth century. The fundamental contrast between East and West, which determined how the shocks of war and disease affected development, was one of maps, not chaps. Justinian’s economy was ticking along nicely—Egyptian and Syrian farmers were more productive than ever, and merchants still carried grain and olive oil to Constantinople—but the West had nothing like the East’s booming new frontier of rice paddies. When Wendi conquered south China he deployed at least 200,000 troops; at the height of his Italian war, in 551, Justinian could find just 20,000. Wendi’s victories captured south China’s great wealth, but Justinian’s merely won poorer and often war-ravaged lands. Given several generations, a reunited Roman Empire might conceivably have turned the Mediterranean back into a trade superhighway, opened a new economic frontier, and turned social development around; but Justinian did not have that luxury."
--Ian Morris: Why the West Rules--for Now