Chapter 1 analyzes the construction of the legal, bureaucratic, and semantic infrastructures of Korean nation-building, which emerged amidst the dramatic transformation of the regional interstate system and the massive intraregional migration in the beginning of the twentieth century. By comparatively examining the colonial state’s engagement with Korean migrants in Japan and Manchuria, Chapter 1 shows how these infrastructures helped the colonial state claim migrants of peninsular origin uniformly as “its own”—if with varying degrees of success—despite differences among these migrants, their resistance to this compulsory incorporation, and the competing claims made by other states. The colonial state’s transborder engagement contributed to the formation of the Korean nation as a legally codified, pervasively institutionalized, and enduringly documented community both inside and outside the colony, providing a critical institutional scaffolding for the diasporic imagination of Korean nationalism and laying the ground for transborder membership politics for decades to come.