According to the Westphalian system of international law, all people are meant to be citizens or subjects of territorially bounded and sovereign nation‐states, which in turn guarantee certain rights to, and impose certain duties upon, their members. Anarchism, by contrast, is predicated upon a rejection of the legitimacy of state sovereignty, and a refutation of the justness and practicability of representative government. Anarchists took individual and collective “self‐determination” to their logical extremes—and in the process confounded state legal regimes and bureaucracies that understood national belonging and individual rights only in terms of citizenship. From the perspective of the United States, alien anarchists “belonged” back in their countries of origin, but from those European states' perspective, anarchists had no place in their national communities. This article examines how both radicals and governments in the era of America's “First Red Scare” engaged with the rules governing the interstate system. As individual radicals, government functionaries, and international diplomats wrestled to define where anarchists belonged in the international order of nation‐states, the solutions they found simultaneously reinforced the boundaries of the Westphalian system and revealed contradictions and fissures within it.