Chinese and Korean protests over “revisionist” Japanese histories of World War II are well known. The impact of contested Chinese and US histories of the Korean War on US-China relations today has received less attention. More broadly, there has been little research seeking to systematically explore just how history textbook controversies matter for international relations. This article experimentally manipulates the impact of nation (US/China), of source (in-group/out-group textbooks), and of valence (positive/negative historical narratives) on measures of beliefs about the past, emotions, collective self-esteem, and threat perception in present-day US-China relations. A 2 × 2 × 2 design exposed randomized groups of Chinese and US university students to fictional high school history textbook accounts of the Korean War. Findings reveal significant effects of nation, source, and valence and suggest that the “historical relevance” of a shared past to national identities in the present has a dramatic impact on how historical controversies affect threat perception.