The process of Unification of three kingdoms was riddled with hardships. Silla, which had least power among three kingdoms, had became close to lose the western part of Gyeongsangnam-do by continual attack of Baekje. Silla tried to find a way out of this crisis and considered Goguryeo as a solution, but it was resultless. In 647, Kim Chun-chu was sent to Wae and it was fruitless, also. Finally, a faithful subject Kim was sent to Tang. Consequently, a contract was made by Kim and Emperor Taizong. According to their ambitious master plan, a discussion about shares after destroying Baekje and Goguryeo was completed. They decided that the southern part of Pyeongyang and the territory of Baekje should belong to Silla. This means that there was a tacit agreement between them; It ment Tang who had the control of northern part of Pyeongyang and Liaodong. As shown in the Yalta Conference and the Potsdam Declaration shortly before the Second World War, territory separation of occupied nations caused many conflicts of interest. At that time, it is obvious that Tang had suggested preemptive rights for old land of Goguryeo. Since Sui dynasty, a unification before Emperor Taizong, Chinese have been emphasizing that Liaodong was originally belong to China. So they set the region to retake, Which we call Hansa-gun(漢四郡) belongs to this area. Tang pointed out old Hansa-gun area of Goguryeo and Riaudongjun of after Wei period as reclaimed areas. What this ment is that there were Hyundo-gun, Jinbeon-gun, Imdun-gun with Lorang-gun. Among them, Jinbeon-gun and Imdun-gun which perished early had no symbolism as well as existence. Hyundo-gun of the third period being located in Musun of LiaoNing Province, they might recognized to be ended due to governing Liaodong. However, Daebang-gun, future being of Jinbeon-gun, seems to have been recognized as a site of Baekje founding. For this reason, it was accepted to admit the rule of Silla that brought up three kingdoms integration theory. In the final analysis, the theory about reclaiming old territory of Tang’s Hansa-gun is very suggestive relating to East-North Project. It was a justification of invasion found in history. Also it is a theory still available to China. Since the unification being established, Goguryeo conquest theory was raised by Emperor Taizong from Sui dynasty upward. We should be aware of that these grounds are latent as a serious obstacle in Korea for the 21st C. Eventually, history is a matter which is already settled ideology that is undeniable.