Nomadic empire
The Xiongnu were a confederation of nomadic tribes from Central Asia with a ruling class of unknown origin and other subjugated tribes. They lived on the Mongolian Plateau between the 3rd century BC and the 460s AD, their territories including modern day Mongolia, southern Siberia, western Manchuria, and the modern Chinese provinces of Inner Mongolia, Gansu, and Xinjiang. The Xiongnu was the first unified empire of nomadic peoples. Relations between early Chinese dynasties and the Xiongnu were complicated and included military conflict, exchanges of tribute and trade, and marriage treaties. When Emperor Qin Shihuang drove them away from the south of the Yellow River, he built the famous Great Wall to prevent the Xiongnu from coming back.
Emperor Ming of Han
As crown prince, Crown Prince Zhuang was often requested by Emperor Guangwu to render opinions in important matters. In 51, he was involved in making a major correct decision in Han's relationship with Xiongnu. By that point, Xiongnu had a civil war and divided into two—with North Xiongnu ruled by Chanyu Punu (蒲奴) and South Xiongnu ruled by Chanyu Bi (比). Han had become allied with South Xiongnu, and in response, Chanyu Punu, also wanting peace with Han, requested a "heqin" marriage. Prince Zhuang suggested that Emperor Guangwu refuse the proposal, reasoning that North Xiongnu had only made the proposal to alienate South Xiongnu from Han. Emperor Guangwu agreed.