Excerpts from Books and Wikipedia
"During the four centuries of Han rule, vast changes occurred in China--not only an increase of population but a growth of the landed estates of local magnates, who gained control of the lands of impoverished peasants when they could not pay their debts and then let them use the land as tenants. . . . Under the most energetic of the Han rulers, the Martial Emperor (Han Wudi, reg. 140-87 BC), Chinese armies penetrated into southern Manchuria and Korea to the northeast and into south and southwest China and northern Vietnam. . . . When militarily weak, which was much of the time, Han emperors used a policy of 'peace and kinship' (heqin)--entertaining the nomad chieftain, giving him Han princesses in marriage, and making lavish gifts, especially silks." [Fairbank: China, p. 60-1]