Owen Lattimore was an early student of frontiers in East Asia, and this paper takes as its point of departure Lattimore’s collection of books and articles relating to Koreans in Manchuria, and the border between Manchuria (then Manchukuo) and Korea. The paper indicates the depth of dependency that Lattimore and others had on Germanlanguage treatments of the border region during the late colonial period, and aligns with the scholarship of Suk-Jung Han in seeking new approaches to reframing the history of interactions along the Yalu/Amnok and Tumen rivers in the 1930s and 1940s.