This Working Paper provides a dual historisation of 'securitisation', i.e. of the origins of the Copenhagen School in terms of its direct world historical context and of the historical origins of the specific bias in our political discourse which is prompted by security discourses. Born almost as a rationalisation of German Ostpolitik, and hence with desecuritisation, the Copenhagen School understood the speech act less as a kind of conspiratorial or elite manipulation than as the manifold processes that give prominence to the discourse of security (the reversal of Clausewitz) in public debate or diminish it, as in the processes of desecuritisation. This means that I see 'securitisation' not in the 'act' of those 'speaking' security, but in the possibly unintended and unconscious de-/mobilisation of the inherent logic, or grammar, of the discourse of security. This begs the question, however, of where the discourse of security would have gained its inherent logic from. It is here where a second necessary historicisation has to take place, not about the context of the theory itself, but about the content of its central concept. The Copenhagen School has been criticised for being basically still too conventional or realist in its reading of security, being connected to exceptional measures, done by foreign-policy elites, etc. But just as the increasing number of security sectors indicates, this is not to be understood as the 'essence' of security, but rather as the effect of a historical development in which certain actors have traditionally come to be authorised to talk and effect war and peace in a 'realist' way. This implies that, by reifying a historical moment into a general framework of analysis, securitisation theory may indeed help to reproduce such an understanding, although it does not need to. In return, it implies, however, that if a different understanding of security (beyond the raison d'État) appears and becomes shared, the Copenhagen School will also have to adapt. Its conceptualisation is historically bound. The Working Paper is the English version of a book chapter in Portuguese, published as: Stefano Guzzini, "A história dual da securitização", in André Barrinha and Maria Raquel Freire, eds, Segurança, Liberdade e Política: Pensar a Escola de Copenhaga em Português, Lisboa: Imprensa de Ciências Sociais, 2015, pp. 15-32.