Recent estimates suggest that more than 3,000 Europeans have travelled to Syria to fight for the ‘Islamic State’ (IS). UK Prime Minister, David Cameron, has argued ‘It is not only the full force of the law that these people should face… when they take up arms in this way in another country, they become enemies of the state.’ In accordance with this view, current counterterrorism policy seeks to curb the citizenship and mobility rights of those suspected of involvement in terrorism. Exclusion orders, flight bans, passport seizure, and forcible relocation are defended as essential to national security. For some, citizenship appears no longer as a right but conditional upon conduct, or a privilege to be diminished or denied. This paper examines these developments and considers the risks to justice, to human rights, and, not least, to security, when citizenship-stripping is used as a tool of counterterrorism. In so doing, it asks what does the duty of the state to protect its people permit?