In 1907, the Second International adopted a resolution on migration that rejected restrictions on the free movement of workers. In this article, we contend that, despite this official stance, the issue of migration was a highly controversial one for the international socialist community. We present a multi-level analysis, in which we detail the migration debate as it took place on the platforms of the Second International (roughly between 1903 and 1907) and the way in which this debate played out domestically for the Sozialdemokratische Partei Deutschlands and the Socialist Party of America – two parties that openly rooted for restrictions at the international level. We discern three ideal-typical stances on immigration – internationalism, on the one hand, and protectionist nativism and xenophobic nativism, on the other – and argue that it was the incompatibility of the internationalist and nativist positions that caused internal divisions to arise during the debates. Apart from speaking to the classic historiography on the Second International, which deals with the incompatibility of internationalism and nationalism, this article traces the influence of additional racist and culturalist ideologies on the debate and further historicizes it within the broader context of the modern international migration system that was taking shape at the time.