1st campaign 128 BC :
Wudi's campaigns against Korea had to do with his worries about a possible alliance between the Huns (Xiung Nu) and the Koreans. Moreover, Ko Choson refused to pay tribute to Han.
Events and Consequences : Chinese submitted NamNyo, Ruler of Ye State (descenant of Yemaek) with his 280 000 men, and created CangHai commandery in 128BC onto Ye territory (between Yalu river and Tongjia river basin). Due to local population resistance, CangHai commandery only lived 3 years (125 BC).
2nd Campaign (109 BC)
Wudi was also unhappy about Choson's cutting off the trade routes between China and the state of Chen (Chenhan or Chinhan) which was on the southern end of the Korean peninsula. In 109 BC, the Han court envoy to Choson, She He, who killed the Korean escort and claimed to Wudi that he killed a Korean general. The Koreans avenged later by killing She He( sent by Wudi in Ko Choson to accept his suzerainity).
Using She He's death as a pretext, Emperor Wudi dispatched two armies against Choson in the autumn of 109 BC, via sea and land, respectively. Unable to subjugate Choson in the first campaign, Wudi sent another envoy to Choson and succesfully pursuaded Korean King into sending the prince to China's court as a hostage. But a Chinese general's attempt of dismantling the Korean prince's entourage aborted the peace efforts.
3rd Campaign (108 BC)
Renewed fighting (60000 men + 7000 seamen under Wudi's order) caused Choson to disintegrate internally. Yok-kye, a Choson minister, fled south to the State of Qin-han with two thousand households including metallurgists, farmers and etc. In the summer of 108 BC, the ministers of Choson King assassinated their king Wei Youqu (Wei Ugo), and surrendered to Chinese. Wei Man Choson and Wang'gom-song fell to Chinese.
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Old Chosun had so grown in strength and domain as to interrupt, in the second century BC, the contact between the Former Han dynasty and the petty walled town states of Chin (the later period Three-Han) located south of the Han River (see Lee, 1984, p. 17). In early 109 BC, the King of Chosun invaded Liao-dong [located east of the Luan River] and killed a Han Chinese officer. Being seriously concerned about the possible alliance with the Xiong-nu, Han Wu-di (141-87 BC) launched an attack on Chosun in autumn. The King of Chosun was killed in summer of 108 BC, and Wu-di established four commanderies, thus "severing the left arm of the Xiong-nu."15
Thereafter, Wudi established four commandaries. In 107 BC, Lelang Commandary was set up; in 106 BC, Xuantu Commandary was set up. The four commandaries, in the order of north to south, would be Xuantu, Lindun, Lelang, and Zhenfan, with Xuantu along the Yalu River and Zhenfan to the south of today's P'yong'yang.