An increasing number of countries, especially the UK, have expanded their legal powers to deprive citizens of their nationality in response to terrorism. The trigger for the mounting focus on this tactic has been the advent of Islamic State and the phenomenon of Foreign Terrorist Fighters (FTFs) travelling to Iraq and Syria. The weaponization of nationality has therefore been adapted to this situation so that citizens can be treated as if they were foreigners. In this way, the FTFs can be divested of citizenship and of rights. This paper will explain these developments against the declining inspiration of ‘cosmopolitan citizenship’. So, the paper first needs to explore notions of citizenship and then to look at the decline of ‘cosmopolitan citizenship’ which provides space for counter-terrorism weaponization. The paper does not contend that cosmopolitan citizenship forbids any weaponization but highlights important provisos which draw attention to human rights and other principled and policy constraints which also must be explored and weighed.