When and how did the Byzantine army transformed from the legion-based late Roman army into one consisting of soldier-farmers and a small cream of professionals?
https://en.wikipedia.org/wiki/Theme_(Byzantine_district)#First_themes:_640s.E2.80.93770s
Basically the deal was to form joint military-administrative units in which soldier-peasants will be tied to land so they can defend it anytime it's needed
Needless to say it worked pretty well in halting Arab invasions across Anatolia
>>1086799
Before the theme, the legion had already collapsed and was steadily replaced with local mercenaries and private guards. The reason was much the same as in the West - the collapse of the annona tribute fleet that supported the standing legionary army, only this time in the East it was Justinian's campaigns and subsequent plague that crippled Roman shipping and taxation for generations, leading to the Sassanid debacle and ultimately the Arab conquest.
Mudslimes
>>1087114
It was really just the plague. The plague was deadlier than the Bubonic Plague and nearly wiped out European civilizations.
Their women stop breeding, just like now.
All the empires fall for the same reason, demographic crisis, that's all there is in history.
