The European security architecture inherited from the period of the Cold War encompasses a few most important international organisations – first of all, NATO, EU and OSCE, members of which are most European countries – and institutional rules as well as numerous informal patterns of state behaviour and status. 2019 is to see the withdrawal of the United Kingdom from the European Union, which is one of the key institutional “axes” of the European security architecture. This will potentially have an impact on the future of the entire organisation, hence – on the security on the old continent. This article aims at compiling a set of alternative scenarios of the evolution of the European security through the use of the scenario building technique which is still bizarre in political science. To this end, interaction of four “driving forces”, namely, 1) USA involvement, 2) threats of regional scope, 3) leadership of Germany (and France) in the promotion of the European integration, and 4) stability of the UK government, in the next seven years, is analysed. Various combinations of these variables lead to the crystallisation of three alternative plots of scenarios: 1) closer European security and defence union, 2) new Cold War, and 3) revival of the global “Anglosphere”. Still, as seen from the practice of application of the scenario building technique, in the medium term, a parallel and only partial materialisation of all three scenarios is most likely.