In the mid-seventeenth century, the Chosŏn Korean (1392–1910) court ran a smuggling enterprise in Japan, using both formal diplomatic agents and private merchants to import military contraband. This royal enterprise caused the largest documented criminal investigation in early modern Japan, complicating diplomatic relations with both the Tokugawa shogunate (1603–1868) and the daimyos of its Tsushima domain. By examining this history of “contraband diplomacy,” this article intervenes in the study of early modern diplomacy and imperialism. Contrary to scholarly portrayals of East Asian polities as isolationist and uninterested in trade, it shows Korean kings and officials aggressively pursuing commerce. If previously misunderstood as “self-organized,” their efforts reveal a different aspect of state formation in the global early modern period. Without becoming imperial or mercantilist, some institutions, such as those of Chosŏn and Tsushima, spearheaded international criminal activities, and others—like the shogunate—exercised growing administrative intent and capacity to stymie them.