One of the perennial questions historians address is why empires fell. In his 1987 bestseller, The Rise and Fall of the Great Powers, Yale historian Paul Kennedy theorized that every empire reaches a tipping point when its military is unable to police the vast amount of land under the empire's control. This "imperial overstretch," in Kennedy's view, caused empires to inevitably shrink and decay.
Kennedy's prime example is China in the 15th century. In the 28 years from 1405 to 1433, a fleet of more than 300 ships led by Admiral Zhang He went on several lengthy voyages, including two to Africa. The four-masted treasure ships, the largest ones of their time, brought spices and exotic animals to China. But after 1433 the voyages stopped. Zhang He had no successors, and China retreated from the world stage and turned inward.
What happened? For Kennedy the Zhang He episode is the classic case of "imperial overstretch." Kennedy explained that the costs of maintaining such a large navy so taxed the Chinese empire's resources that the state could not afford to maintain the costs of such a huge fleet. Kennedy felt that, after Zhang He's last voyage, the Chinese thought that a large fleet was too costly.