Dates That Made History S03 E15 221 BC - Founding of the Chinese Empire

In 221 BC, the king of Qin, a small state on the western edge of China, defeated the Warring States period and unified the Chinese Empire. Tradition has made this first emperor the founder of a political system that presents itself as immutable. But when did this date come into being? Since when has 221 BC been considered the year zero of imperial China?