How did the barbarians from Germany and the east eventually defeat the Romans?

Answer 1

The composition of the army changed from a citizen militia to a professional army, to a force mostly consisting of recruited barbarians. Once the barbarians learned Roman tactics, they were able to use that knowledge to defeat the Romans when they turned on them. There was also a great deal of political in-fighting between the Roman commanders who cared more about personal glory than with the safety of the state.

Answer 2

The Germanic tribes invaded the western part of the Roman Empire. The Romans lost political cohesion. They were busy fighting each other they and were unable to respond to the invasions effectively. One emperor undertook successful campaigns against them, but then he was deposed by a political rival. Once these the invaders became entrenched in the areas they occupied, they became powerful.

Answer 3

Because the Roman army was more disciplined and its weapons were technological more superior.