Why did Byzantium escape ruin from outside thieves?

This is due to a number of reasons:
1) Byzantium had strong political power;
2) The lands of Byzantium were out of the reach of the barbarians. So, in order to fight with Byzantium, the barbarians needed to build a strong fleet, but the barbarians had neither the knowledge nor the resources for its construction;
3) Byzantium had a strong army, which made it possible to fight an external enemy;
4) Byzantium itself suffered less from the Hunnic invasion than the Western Roman Empire.