The purpose of this paper is to examine development and implication of the theory of Shilla’s unification since modern times. The theory of Shilla’s unification varied according to the discourse construction selected among nation, state, and territory. The theory of Shilla’s unification during the Japanese colonial period will be discussed first. Japan’s modern view of history introduced the concept of Shilla’s unification with modern meaning. The theory of Shilla’s unification developed in two directions with different implications in the identical category of colonial view of history. The Theory of Han's unification in Shilla did not correspond with Japanese colonial policy promoting Korea's assimilation into Japan. An official opinion from colonial view of history was the theory of peninsula unification based on state and territory. It intended to include Balhae, north of the Daedong River, into history of Manchuria by presenting Shilla unified only south of the Daedong River in 675. However, the border between Shilla and Goguryeo formed around the Imjin River then. The following is the theory of Shilla’s unification after liberation. The absence of the state and ideological confrontations were realistic background to the theory of national unification based on nation and state developed. However, it only expatiated on national meaning of unification, failing to criticize the theory of peninsula unification founded by Japanese colonial view of history. Shilla did not include north of the Imjin River as its territory until 735 when it dominated south of the Daedong River, although it is claimed Shilla unified the Three Kingdoms by expelling the Tang Dynasty in 676. The perception of history between South Korea and North Korea has differed deeply due to authenticity since the Korean War. North Korea, from the viewpoint of nation, denies Shilla's unification of the Three Kingdoms, but South Korea acknowledges Shilla's unification from a standpoint of state. In conclusion, it is more important than anything to understand a basic fact Shilla integrated Baekje and Balhae was founded on Goguryeo's old territory, when interpreting results of international wars in East Asia in the late 7th century.