As the Roman Empire established itself, the mostly Germanic
hunter-gatherer tribes beyond the Rhine and Danube rivers, the
northern and eastern borders of the Empire, were settling into
an agricultural way of life. The inevitable population explosion
that follows the agricultural revolution drove the Germanic
tribes into desperate movement to the south. For centuries,
their immigration was slowed by far-superior Roman forces to a
flow that was absorbed by the existing societies of the Empire,
much like current immigration across the southern border of the
United States. However, during the time of Constantine and the
following century, the pressure of barbarian numbers and
aggression increased dramatically even as the Roman ability to
contain them declined. The decline was due to much more than the
diversion of military forces to the East (with the Emperor’s
move to the Greek city, Byzantium, later called Constantinople
and the center of Eastern Christianity). Thomas Cahill describes
with convincing insight,