In the north, the Five Barbarians established numerous kingdoms, leading to the period being known as the Sixteen Kingdoms. Eventually, the Northern Wei conquered the rest of the northern states in 439. Although the Eastern Jin and successive southern dynasties were well-defended from the northern states by placement of naval fleets along the Yangtze, there were still various problems faced with building and maintaining military strength. The designation of specific households for military service in the tuntian system eventually led to a falling out in their social status, causing widespread desertion of troops on many occasions. Faced with shortage of troop numbers, Jin generals were often sent on campaigns to capture non-Chinese people in the south in order to draft them into the military. The Eastern Jin dynasty fell not because of external invasion, however, but because General Liu Yu seized the throne from Emperor Gong and establishing himself as Emperor Wu of Liu Song (r. 420–422), which officially began the Northern and Southern dynasties.