Under Ming emperors, China extended its rule into Mongolia and central Asia and briefly reconquered Vietnam. During the late sixteenth century, internal power struggles developed. Groups worked to gain sole power and to place one of their leaders as emperor, their efforts resulted in a series of weak rulers who were overpowered. Children were sometimes placed on the throne. Internal power struggles led to a period of government corruption, high taxes caused by the corruption led to peasant unrest, crop yields declined because of harsh weather. In the 1630's a major epidemic greatly reduced the population in many areas. The suffering caused by the epidemic helped spark a peasant revolt led by Li Zicheng. The revolt began in China and spread to the rest of the country. In 1644 Li and forces occupied the capital of Beijing. When the capitol fell the last Ming emperor committed suicide, many officials took their own lives as well. The overthrow of the Ming dynasty created an opportunity for the Manchus. They were farming and hunting people who lived Northeast of the great wall in the area known today as Manchuria. The forces of the Manchus conquered Beijing, and Li Zicheng's army fell.