Map of the Roman Empire at its greatest extent in 117 A.D. under Emperors Trajan and Hadrian.
The Roman Empire reached its greatest size at the beginning of the second century AD. By then the empire was so large that it became increasingly difficult and expensive to manage and defend its borders. The barbarian hoards continuously pushed upon the northern and eastern boundaries, discouraging the border economies which were further discouraged by the heavy taxes required to maintain the army.
The end to incremental territorial conquests meant a sudden stop to the huge influx of wealth and (cheap) slave labour. All these factors went hand in hand with the Empire’s decline.
The empire had reached such a size that it eventually had to be split into two manageable halves, West and East. The western half eventually succumbed to successive barbarian invasions from around 475AD onwards. The picture left shows Italy and the capital of the West: Rome.
The Eastern half with its capital at Constantinople (Bizantium-Istanbul) survived through to the 15th Century when its walls were breached first by the Venetian crusaders and then by Turkish cannons. The picture shows the Eastern half and its future capital marked as “Byzantium”.