During the last decade, small groups of countries have taken a number of initiatives to establish or re-vitalize defence cooperation in Europe. These initiatives have often been launched outside of the Common Security and Defence Policy (CSDP) and NATO. The purpose of the study is to highlight different forms of bilateral and regional defence cooperation and their possible effects. The Franco-British defence treaties from 2010 serve as a case study. The positive effects of bilateral and regional defence cooperation for the involved countries include enhanced effectiveness and efficiency, additional synergies, a preserved or even developed capability, enhanced interoperability as well as less duplication. Well-functioning bilateral or regional defence cooperation is expected to entail greater effects in comparison to both multilateral cooperation and initiatives at a national level. The possible effects on the common security in Europe are multifaceted. Strengthening defence capability within a group of countries should be favourable for Europe as a whole. However, if defence cooperation is gradually concentrated to bilateral and regional associations a possible effect may be that less time and energy is devoted to the collective capability and shared security at a European level. Coordination, openness and information exchange between the different collaborations will be key. The effects of the different defence cooperation forums and initiatives will to a great extent depend on how the countries in Europe choose to deal with the current situation and how, where and with whom they choose to develop defence cooperation.