The purpose of the present study lies in elucidating the Sinchon Incident, which occurred in Hwanghae Province during the Korean War (1950-1953). The incident has been defined as the “October 13 Patriotic Anti-communist Incident” and the “Sinchon Massacre” by American Forces in South and North Koreas, respectively, and used to propagandize anti-communism and anti-Americanism, respectively. The present study reviews the prevalent views on the Sinchon Incident in the two Koreas. By grasping the implications and problems of the opposing claims of the two nations, it seeks a transition in the perspective and method of looking at and addressing the incident. Next, it examines the situation in the Sinchon area during the war by dividing it into periods including those impending occupation by the United Nations (UN) forces—i. e., the UN forces’ occupation of Sinchon and North Korea’s retreat—retreat of the UN forces, and reoccupation by North Korea. The present study understands the Sinchon Incident in relation to national division at the 38th parallel north, war situation, and, in particular, the UN forces’ advance beyond and retreat from the 38th parallel and the consequent repetition of occupation by either side. First, the “October 13 Patriotic Anti-communist Incident” and the “Sinchon Massacre” were not an identical (contemporaneous) incident but were two interconnected incidents. While South Korea stresses the nature (patriotic anti-communist incident) and the agents (North Korean refugees) of the incident that took place before the occupation of Sinchon by the UN forces (United States forces, in fact), North Korea highlights the incident that occurred after occupation by the American forces. Second, there is the dual nature of the period before the American forces’ occupation of Sinchon, or October 13-17, 1950. While the October 13 incident was simultaneously a preemptive uprising against North Korea’s preventive custody and a patriotic anti-communist incident, it was also a process that led to the massacre. Third, the massacre committed during the 52 days that North Korea calls “temporary occupation” (evening of October 17-December 7, 1950) took place not so much due to the American forces’ continued and direct orders but rather due to the absence of a system regarding occupational rule. There existed no measures whatsoever to prevent massacres such as laws and instructions, and executions were performed according to the judgments and feelings of individual members of vigilante groups, which consisted mainly of local anti-communist, right-wing figures. Fourth, the massacre occurred with the repetition of occupation by either side because of dramatic changes in the war situation, or advance beyond and retreat from the 38th parallel. The northward advance of the UN forces and its pressure led to North Korea’s retreat and preventive custody, and following the retreat of the Korean People’s Army (KPA; North Korean army), anti-communist right-wing vigilante groups killed the “commies.” As the KPA came to reoccupy the area and the UN forces came to retreat south of the 38th parallel, the vigilante groups massacred, immediately before retreat, those they had imprisoned, and the KPA killed “reactionary elements.” The so-called “reactionary elements” and “commies” as well as those considered fellow travelers were killed by both sides under the pretext of retaliatory executions.