One of the nice things about reading history is the discovery of new things, the way it generates new ideas. The ideas may not be new in an absolute sense, but they are new to the reader.
I have mentioned before just how closely Harry Turtledove based his Videssos series on Byzantium. It is Byzantium with religions changed and the addition of magic.
There are a number of places in the series where Turtledove refers to the forced relocation of farming peoples.
In one case, the invading nomads seize farmers and forcibly relocate them to one of their own farming areas. They detest farmers, wrong life style, but need their produce. Videssos finally buys them back, resettling them in another area. Krispos, the young hero, asks his father why they are not returning to their own farm. His father explains that other people will have been settled on their land.
In another case, the Emperor is fighting against a group of religious fanatics, a splinter from the main church. Upon defeat of the rebels' army, the decision is made to forcibly break the population up and re-settle them in different parts of the Empire. Their place will be taken by peasants loyal to church and state.
In Turtledove, I noted this as part of the story. I don't think that I realised until reading Treadgold on Byzantium just how important this resettlement process was.
The first key is transport costs.
Writing of Australia, Geoffrey Blainey notes that transport by land was twenty times more expensive than by ship.
It cost more in 1820 for a Sydney merchant to send a barrel of whale oil (whaling was then a major Australian industry) 100 miles inland than around the world to London. This meant that only commodities that were extremely valuable on a per ton basis could afford transport from areas more than 40 miles from deep water.
This was even truer in the period we are talking about. Byzantium and the earlier Roman Empire before it needed food to feed its cities and towns and to support its armies. Initially food was shipped from the granaries of Eygpt and North Africa, taking advantage of sea transport. Following the loss of Eygpt, Byzantium relied upon other parts of the Empire closer to the capital to feed its people.
Agriculture needs farmers. This introduces the second key, population change.
From time to time, the Empire was devastated by disease. Famines caused by drought and pests such as locusts as well as war could depopulate individual areas in whole or part.
Say the word plague and those brought up on Western History think Black Death. This outbreak affected Byzantium in its last days, but in the huge expanse of Byzantium history it was only the latest of a series of outbreaks that had taken place with gaps over the previous centuries.
The third key is security.
Throughout its long history, the Empire was constantly pressed not just by rival Empires such as the Persians and then the emerging Western European states, but also by the constant arrivals of new peoples looking for land and plunder.
For its part, the Empire was looking to expand, to recapture territory lost. The dream of re-building the Roman Empire exercised a continuing fascination over the centuries, as indeed it did in the west. Rome did not vanish - it continued to affect the pattern of life to the present time.
These three keys dictated what today we would call population policy.
Depopulated farming areas were resettled to grow food and to provide security. Land grants were made to support locally based military forces drawn from different parts of the Empire and beyond. Invaders were fought, but sometimes also settled within the Empire's boundaries. Whole peoples from client states or outer parts of the Empire under threat might be relocated to another location.
As the Empire's boundaries expanded and contracted, people were lost and gained. As boundaries contracted, the composition of the peoples beyond the frontier changed, in some cases permanently. As boundaries expanded, new peoples were incorporated in the Empire, changing people composition within the Empire.
This process extended over centuries, continuing into the Ottoman period. One outcome was the creation of an ethnic patchwork quilt, a complicated pattern that continues to exercise influence today.