What factors contributed to the weakening of the empires frontiers?

Answer 1

One factor was the length of the vast frontiers of the empire. In Europe they stretched from the mouth of the River Rhine, on the North Sea, to the Black Sea. Another factor was the lage number of peoples who lived on the other side of the frontiers.

In the third century, these two factors led to the Roman army being overstretched. There were numerous raids, incursions and invasions by the many neighbours of the Roman Empire in Europe. These occurred both on the Rhine frontier and the Danube frontier, at great distances from each other. The Romans troops often had to be redeployed to where an attack took place. This often led to a cat and mouse game, where when the Roman troops were redeployed to one frontier, other peoples on the on the frontier took advantage of this to carry out raids until the Roman army was redeployed to this other frontier. This was the particularly the case with the peoples along the Rhine frontier. The strain of this situation led to a period of military anarchy where many emperors were military commanders who were proclaimed emperors by their troops. Many were murdered, even by their own troops.

The emperor Gallienus created heavy cavalry units which acted as rapid deployment forces which could move to the areas under attack quickly. They were stationed at several frontier points. Their headquarters was in Milan because of this city's strategic location. It was not far from the vulnerable areas of the frontiers: the Alps, the Rhine and the Danube.

Another factor was population pressure on the land north of the Roman Empire. There were southward migrations to central Europe which squeezed the peoples who live there. The Romans allowed the Franks, who lived at the mouth of the River Rhine, to settle in their empire, in Toxandria (Holland south of the Rhine and parts of northern Belgium) as allies of the Romans. This was because the Franks were being squeezed by incoming peoples. They also allowed the Visigoths to settle in the lower Danube area of the empire because they were seeking a sanctuary from the invasion of their homeland (Ukraine) by the Huns. In the 5th century, the population squeeze led to several Germanic peoples invading the western part of the Roman Empire because they were seeking new lands to settle.

A factor which has sometimes been seen as a weakening of the Roman army was its increasing reliance of Germanic soldiers, either those of the allies or mercenaries in the late 4th century and in the 5th century.