Barbarian Invasions
Stoicism and now the Christian doctrines which absorbed it, did not encourage independent thought, independent action, or independent learning. Rome had flourished in large measure under the Greek culture and spirit of curiosity which it imported, certainly the most important spoils of war which Rome brought back from any military campaign. Combined with the Roman preoccupation with personal responsibility and a shrewd, practical streak, these talents had taken the Roman Empire across the entire Mediterranean, the only power of any era to accomplish that feat.
However, the downfall of the Republic undercut these successes and changed the structure of Roman power. The vaunted Roman armies gradually became less and less effective suffering their first massive defeat in 9 AD against the Germans. As the Ancient Era drew to a close it became customary for Rome to supplement its strength with Germanic armies to guard large sections of the empire. There was another reason, however that the Romans found it necessary to augment their military strength – barbarian invasion.
In 9 AD, the Germans had been brutal barbarians. By 300 AD, they had founded kingdoms and developed politically and culturally under Roman influence. However, they were under pressure from other barbarians invading from Asia. All this had been set in motion by the wars of the Chinese Han Dynasty against the Xiongnu barbarians, and when the Xiongnu were chased out in 100 AD, they started a rolling tidal wave of barbarians aimed directly at Europe. Barbarian refugees who'd been chased from their homes by more powerful tribes routinely petitioned Rome for sanctuary within the Empire in exchange for their military service. Rome, which was under pressure from the same barbarian invasions made a virtue of necessity; thus its army was largely built on the backs of these "federeated" Germanic tribes. Then, in 372 AD, the Xiongnu themselves invaded Europe, now known to history by their Roman name, the Huns.
Attila the Hun ravaged much of Europe and was the single most dangerous threat to the Roman Empire, but the real problem faced by the Romans was all the competition the Huns faced from other barbarian tribes. The sheer volume finally overwhelmed the Romans and after their defeat at the Battle of Adrianople in 378 AD, their German allies began to understand it. Thus the end of the Roman Empire did not come in a great battle or a deadly assassination, but in the decision of Rome's allied tribes to ignore it and rule the Roman Empire for themselves. Beginning in 406 AD, the tribes began carving up the Roman Empire and in 410 AD, Alaric, King of the Visigoths, sacked Rome itself. It was the first time the city had fallen since the sack by the Gauls in 387 BC. The Roman Empire's disintegration spelled the end of political unity on the continent for the rest of the Medieval Era.
On the one hand this seemed counter intuitive, and certainly to Renaissance historians it was almost inexplicable that the mighty centralized Romans would collapse while the divided, decidedly less sophisticated Germans would survive. A typical explanation was to inaccurately ignore the many Roman allies and lay the blame on all the barbarians whose invasions destroyed Rome by sheer numbers. Clearly this does not jive with the reality on the ground. Many Germans ruled simultaneously on the very foundation of Roman authority - at least until they realized that foundation was no longer important to their continued success.
Roman political institutions had slid backward, and could not survive under the mounting pressure. It's true that the Germans lacked the type of bureaucracies which were so successful during the Ancient Era. They depended on feudalism, a form of cooperative rule where each ruler created a type of contract with a group of subordinates that ruled for him. These subordinates chose their own subordinates, etc. creating a government that gained many of the practical advantages of limited and local rule. At the same time, unlike the Ancient bureaucracies, feudalism still theoretically placed the monarch in total control, a pattern that meshed beautifully with the type of influence a ruler could exert through the intelligent use of religion.
For the next century, German armies raged back and forth across Europe until some stable borders began to emerge around 507 AD. Contrary to Renaissance historians, every major kingdom was not established by barbarian invaders but by the federated tribes on whom Rome's survival had depended. Then, in 607 AD, the heirs of the Roman Empire faced their first major challenge, the Juan Juan Confederation which was driven out of China about 50 years earlier. Known to Europe as the Avars, they were the most vicious central Asian barbarians since the Huns and they founded a brutal central Asian style Khanate in the middle of Europe (roughly centered on Southern Germany). For the next 200 years, they would raid Europe ruthlessly. They had competition from the Arab corsairs, pirates who pillaged and slaved along the Mediterranean coast and using rivers as highways, penetrated deep into the heart of Europe from 650 AD on.
In 791 AD, the Frankish king, Charlemagne, crushed the Avars ending the threat they posed to Europe and laying siege to the Muslim cities of Northeastern Spain. He died too soon to save Europe from its newest threats. In 793 AD, the Vikings made their first appearance in history, raiding and pillaging in England and Ireland. For the next two centuries Vikings would not only raid Northern Europe and the Mediterranean, they would settle states in Northern France, seize England for several generations, conquer Sicily, and found Kievan Rus, the seed of what would become Russia. They were so feared, that the Emperors of Constantinople created a new elite unit, the Varangian Guard, which was charged with personally protecting the Emperor, and membership was limited to Viking warriors only. Finally, in 899 AD, the Magyars invaded and terrorized Eastern Europe. By 1000 AD, Europe had suffered through almost 800 years of uninterrupted invasions from Asian barbarians and the Islamic powers of the Middle East.