When an empire falls and leaves no leadership, a power vacuum forms. This is when people fight for control of a specific area.
In Europe and China, after Rome and the Han fell, the governing structures of the empires and trade networks collapsed.
In Europe, this led to the rise of local, self-reliant, tribal governments and the early medieval manorial system. When the Han Dynasty fell in China, the empire was split into smaller and even smaller independent kingdoms later.
Trade in China remained under threat from raids by Mongols and Xiongnu. In Europe, trade was limited to manorial exchanges between manors.
Religion was a unifying factor in Europe and China at this time. In China, Buddhism became popular, while Christianity united communities in Europe.
As people rebuilt, they adapted to their new realities, forming new social structures and relationships that would set the stage for future developments in both regions.