This is a detailed history of the Germanic and Slavic people who invaded
and eventually populated Europe. Heather shows how the Germans were attracted
by Roman wealth, benefited from trading with Rome, then eventually overthrew
Rome. Their initial impulse was the Hun invasion that pushed them west, but
at the same time it was Rome that indirectly helped them develop militarily
and economically. Ditto for the Slavs: on one hand the Avar invasion pushed
them into the lands abandoned by the Germanic tribes (Eastern Europe), and
on the other hand the wealth of the Frankish empire, of the Byzantine empire
and of the Arab empire worked as a magnet.
Then came the Magyar invasion a few centuries later and history did not repeat
First of all the people who lived there (the Moravians) fought back instead
of simply moving west. Second, the invaders settled down (they did win the war).
Barbarian progress towards military triumph and statehood was largely due to economic and military development derived from living near an empire; client states turned into rising powers and eventually enemies of the empire. Invasions from Asian tribes created the stimulus, but the dynamics was already in place and would have worked out anyway, sooner or later.
Peter Heather closes the book with history's equivalent of Newton's third law: "The exercise of imperial power generates an opposite and equal reaction among those affected by it until they so reorganize themselves as to blunt the imperial edge".