In 221 BC, China became an empire when the State of Qin ended the chaotic Warring States period through its conquest of the other six states and proclaimed the Qin Empire (221–207 BC). The Qin Empire is known for the construction of the Great Wall of China and the Terracotta Army, as well as the standardization of currency, weights, measures and writing system. It laid the foundation for China's first golden age, the Han Empire (202 BC–AD 9, AD 25–220). The Han Empire expanded into Central Asia and established trade through the Silk Road. Confucianism was, for the first time, adopted as an official state ideology. During the reign of the Emperor Wu of Han, the Xiongnu were pacified. By this time, only four empires stretched between the Pacific and the Atlantic: the Han Empire of China, the Kushan Empire, the Parthian Empire of Persia, and the Roman Empire. The collapse of the Han Empire in AD 220 saw China fragmented into the Three Kingdoms, only to be unified once again by the Jin Empire (AD 266–420). The relative weakness of the Jin Empire plunged China into political disunity that would last from AD 304 to AD 589 when the Sui Empire (AD 581–618) reunited China.