How did Greek and Roman cultures survive the germanic invasions?

The Militaristic, Cultural, Economical, and geological advantages of the Greek and Roman Civilizations.

Explanation:

The Germanic tribes weren't nomadic, they expanded rapidly with land-hungry peasants.  They had skilled leaders despite their reputation for being 'savages' or 'brutal'.  The Greek and Roman cultures had advanced civilizations with centuries of military training.  They easily fended them off for generations.  Not only did their military and wealth advantages help but the terrain as well, the bodies of water, for example, the Black Sea provided a geological barrier, as well as the Mediterranean Sea.

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