The idea that internal inclusion requires external exclusion features prominently in many theoretical accounts of modern states and citizenship. In a similar vein, it has been argued that internal freedoms of the European Union require strict immigration control at the external borders. This is a widespread perception amongst EU politicians and scholars of European integration. Despite the high salience of border management on the EU’s political agenda, there is surprisingly little empirical research on the relationship between inclusiveness and exclusiveness in the attitudes of European citizens. To bridge this gap, we elaborate theoretically on the link between public support for free movement and the perceived threat from non-EU immigration. Europeans are expected to regard open borders within Europe as a threat if they think that the EU’s external borders are insufficiently controlled. The argument is tested in the setting of the refugee crisis of 2015 using synthetic control method to estimate causal effects based on Eurobarometer data from 2012-18. Our findings speak both to the literature on public opinion in the EU as well as to current debates on public attitudes towards migration.