Answer:
The disintegration of the Western Roman Empire in the 5th century CE created a power vacuum in Europe that was eventually filled by the rise of smaller Germanic kingdoms. These kingdoms were often in conflict with each other, and this internal strife helped to prevent any one of them from become too powerful.
In contrast, the Eastern Roman Empire (better known as the Byzantine Empire) remained strong and unified. China, meanwhile, was unified under the Tang and Song dynasties (618-1279 CE) and experienced an era of unprecedented economic and cultural growth. However, the Song Empire was eventually conquered by the Mongols in the 13th century CE, after which China entered a period of political and economic decline.