National security and national identity exist in a dialectic relationship. Effective national security depends on defining both who the people are, and who they should be protected from. Similarly, the Westphalian system of nation-states relies on the permanence of borders separating citizens from undesirable outsiders. This exclusionary directive of nationalism is evident in the way British counter-terrorism policy and legislation is based on a rationale that frames terrorism as a problem of the Other. Consequently, British counter-terrorism strategy acts as a pervasive form of border control, regulating not passports of visas, but behaviour and belief. This paper will analyse both counter-terrorism policy and legislation to show how a national security strategy is a tool in regulating membership in the United Kingdom, serving to actively redefine what it means to be a British citizen.