Introduced to Korea around 1900, the modern idea of the ethno-nation (minjok) developed into one of the most important intellectual and political concepts circulating in the country by the early 1920s. From the nationalists' viewpoint, the ethno-nation, seen as an unchanging and homogenous entity, was the primary site for individuals' belonging. The national collectivity was a prerequisite for individuals' existence. While nationalists had been celebrating a primeval, immutable and rather ahistorical "Korean-ness" since the last precolonial decade (the 1900s), the Marxists—strongly influenced by Otto Bauer's and Joseph Stalin's understandings of nation as a product of capitalist modernity—started to question the nationalistic approach to Korean identity as a matter of principle by the late 1920s and early 1930s. There was no full agreement among them on how to understand the history of the Korean ethnonation. Some of them believed that the Korean ethnic core dated back to the age of the Three Kingdoms (the first century BC to AD 668). Others put heavier emphasis on the role of proto-capitalism and markets in the modern development of national consciousness, tracing this development to the seventeenth or eighteenth centuries. This article summarizes these debates—between nationalists and Marxists, and also within the Marxist milieu—and links them to Marxist intellectual developments elsewhere. The author argues that the "proto-constructivist" approach articulated by some colonial-age Marxists was an important counterweight to the nationalist nativism of the 1920s and 1930s and, in the end, made a significant—and still largely unappreciated—contribution to the development of scholarship on Korea's history and culture.