Studies of European colonialism have long documented how colonial states served as incubators of nationhood, yet the literature has limited its analytic scope largely to the encounters and ethnic mixings that took place within the territorial boundaries of colonies. This article examines a hitherto understudied phenomenon, the colonial state's trans-border engagement with its subjects who left the territorial unit of the colony and its impact on the contested development of diasporic nationhood. My empirical focus is the shifting trajectories of the classification struggles over Korean migrants in Manchuria during Japan's occupation of Korea. I identify the tumultuous and uneven development of specific legal, organizational, and bureaucratic infrastructures that helped the colonial state extend its trans-border reach and define and identify these migrants as “its own,” often against suspicion, sabotage, hostility, and resistance on the part of other states, indigenous populations, or migrants themselves. I argue that the colonial state's extensive and intensive transborder engagement provided a critical institutional scaffolding for the imagined community of the Korean nation, which came to be conceived as transcending the geographical boundary of the colony. This article contributes to the comparative studies of empire, migration, diaspora, and nationhood formation by challenging the prevalent sedentary bias of the existing literature, by elucidating the critical infrastructural underpinning of the formation of diasporic nationhood, and by extending the horizon of comparison to the political dynamics and long-term ramifications engendered by the migration of, not only metropolitan settlers, but also colonial subjects, within and beyond the ambit of the empire.