it's late, and I've been up since 330am yesterday...
After the succession wars, in which legions proclaimed their commanders emperor and proceeded to slug it out against one another, the romans had to go from active border maintenance to "defense in depth"
after being overrun by barbs too often the outlying provinces nucleused around fortified towns and the lands became less profitable to the governors as tax generators.
so, the governors began allowing unassimilated barbs to settle in the depopulated provincial areas, provided they pay taxes.
this sped the deromanization of the provinces and led directly to the western empire's fall into the dark ages.
from wikipedia:
"By the end of the Crisis of the Third Century this stagnation of Italy could easily be seen in the provincial born Emperors such as Trajan and Hadrian. Economic problems only grew for Italy as time continued, and it eventually spread into much of the rest of the west, especially Gaul, whose industries, particularly the pottery industry, began to suffer tremendously as the 4th century wore on. Another key problem that was almost certainly related to the economic slowdowns in Gaul and Italy was the cost on manpower of maintaining the frontier. Illyricum, a province technically part of the East but more or less in between Rome and Constantinople also suffered heavily in this regard as barbarian invasions increased in the Late Empire in strength and frequency."
http://en.wikipedia.org/wiki/Western_Roman_Empire
"The Romans not only hired mercenaries but also made treaty with tribes that should pacify parts of the empire especially in west. So the migration was partly more or less planned migration. Rome normally could pay the laeti / mercenaries and they were trustworthy as long as they get paid. But later when local chieftains/ kings hired them it was worse. If they did not get paid they naturally were forced to get food from somewhere."