Chain reaction

Ancient China suffered from raids from the Xiongnu Confederation of modern-day Mongolia. At first, the Chinese tried defensive tactics such as building the Great Wall and paying tribute donating gifts. But the raids continued, so in 119 BCE the Han Dynasty sent an army into Mongolia. The Chinese army was much smaller, but better trained and much better supplied. In three battles they pretty much annihilated the Xiongnu army, with the survivors splitting into smaller groups (and some even becoming Han vassals).

Some Xiongnu tribes retreated north and west into modern-day Siberia and south Russia, pushing the residents westward. They, in turn, moved into eastern Europe and Germany. This pushed the Visigoths of Germany southwards, where they sacked Rome in 410 CE. While Rome was no longer the capital of the Roman Empire, it was still a major cultural and economic centre, and its fall sent shockwaves across Europe.

The migrations also caused the collapse of the Burgundian Kingdom (in modern-day Rhineland, not modern-day Burgundy) at the hands of the Huns (and their Roman allies), which forms the backdrop of the German epic the Nibelungenlied (not to be confused with Wagner's Ring of the Nibelung, which, despite sharing most characters, borrows heavily from the Norse version of the legend and Wagner's political (socialist) and religious (Buddhist) beliefs).