Shi Huangdi unified China under the Qin dynasty in the 3rd century BC, establishing the first centralized imperial government and initiating large-scale projects like the Great Wall and his massive tomb. The Han dynasty restored order after the Qin and expanded the empire through the 2nd century AD under emperors like Gao Zu and Wudi, who strengthened the government, economy, and Confucian ideology while also opening the Silk Road. The Han era saw advances in science, technology, and medicine establish China as the most advanced civilization of its time before its eventual collapse in the 3rd century AD.