[FreedomCalls]

Don't forget the illegal immigration. From the Vandals, the Visigoths, the Ostagoths, the Alans, and so forth. The Visigoths in particular asked permission to cross over into Roman territory to escape other tribes pressing them from the east. Romans were OK with it because they only wanted jobs and land that Romans did not want. They ended up deposing the Roman Emperor and ending Roman rule a few decades later.

[haroldeveryman]

As the empire’s ability to defend its borders declined the Romans allowed barbarian tribes to settle border areas. This gave them a vested interest in defending the borders against raids by other barbarians. These Germans increasingly manned the regular western Roman armies, initially as grunts, and in time as generals. The western part of the empire broke up when one of these German generals (Odacer) informed the emperor that his services were no longer required and declared himself King of Italy. Thus, the Western empire went out with a wimper. Shortly thereafter the Eastern emperor Justinian reconquered Italy, Spain, and North Africa, but the western provinces were in such a sorry state economically that they was nothing but a financial drain on Constantinople and were gradually abandoned.