Sima Yi's Liaodong campaign occurred in 238 CE during the Three Kingdoms period of Chinese history. Sima Yi, a general of the state of Cao Wei, led a force of 40,000 troops to attack the kingdom of Yan led by warlord Gongsun Yuan, whose clan had ruled independently from the central government for three generations in the northeastern territory of Liaodong (present-day eastern Liaoning). After a siege that lasted three months, Gongsun Yuan's headquarters fell to Sima Yi with assistance from Goguryeo (one of the Three Kingdoms of Korea), and many who served the Yan Kingdom were massacred. In addition to eliminating Wei's rival in the northeast, the acquisition of Liaodong as a result of the successful campaign allowed Wei contact with the non-Han peoples of Manchuria, the Korean Peninsula, and the Japanese archipelago. On the other hand, the war and the subsequent centralisation policies lessened the Chinese grip on the territory, which permitted a number of non-Han states to form in the area in later centuries.