Dear Barry,
Forgot to mention yesterday that the "demographic sink" notion bears
not only on the rise of Islam but on that of the Slav states in the
Early Mediaeval Balkans. The Bubonic Plague killed a larger portion
of the population in regions which were warm and moist climatically,
or were fairly highly urbanized, than it did peoples in hot and dry,
or colder, climates and which were comparatively sparsely urbanized.
As populations decline, relatively, behind Byzantine frontiers and
state resources become incommensurate with defending them, "Barbarian
Invasion" threats tend to build up. As they had during and after the
smallpox-measles pandemics. One solution to the problem, in theory,
is to admit migrants to compensate for labour depletion. Which is
always tricky; ask Emperor Valens (d. 378) about that. Of course, he
was a fool whose regime was suicidally crooked. But the same was true
of other rulers who admitted Germans and Slavs, and had no means to
keep mobile Saracens out.
The problem becomes hopeless when even a competent political regime
cannot defend all its frontiers with the available resources. This is
what happened to Emperor Maurice (580-602), who was finally killed by
his own weary troops. Enemies which cannot be defeated permanently
because they lack cohesive state structures induce "political fatigue"
in armies which are coup-prone, anyway. By the time the Persians invaded,
during the "superpower showdown" after 602, and especially after about
615, the Balkans had been already overrun.
Sincerely,
Daniel A. Foss