Why were the Romans able to defeat the strong neighboring states, but were defeated by those who did not have state guilt?

The Roman Empire has been building up its power for several centuries. She attacked the states and ruined them. And she appropriated the looted property and slaves. Her offensive tactics made it easy to defeat any country. Therefore, the territory of Rome grew rapidly and replenished with new resources. With progressive development, the Roman Empire ceased to improve the military sphere. The heyday of Roman luxury and prosperity began. The army gradually fell into decay. Therefore, stateless barbarian tribes destroyed the empire.