The thesis results from a research project, combining elements of European law and public international law. The project focuses on the different forms of the use of force by the European Union in the sphere of the Common Security and Defence Policy as an integral part of the EU’s common foreign and security policy. It examines the conditions under which the European Union can engage in military crisis management missions from the perspective of European Union law as well as from the perspective of public international law. The main emphasis of the thesis is put on the former, analysing the EU’s ambitions to become an international security actor from an inside-out perspective. When addressing the vertical dimension of the EU and the use of force in more detail, the thesis analyses the extent to which the Member States are constrained in the conduct of their national foreign and security policy through decisions by the European Union in the sphere of the Common Foreign and Security Policy. With regards to the EU’s legal relationship with the United Nations, the thesis examines whether and if so to what extent the European Union, although not a member of the United Nations, is bound by UN Security Council resolutions in respect of the use of force. Based on the assumption that the EU is bound by UN Security Council resolutions imposing economic sanctions, the thesis uses a comparative method in order to show that the EU as an international organisation is bound by decisions of the UN Security Council in the sense that the EU is obliged to respect the wording and limits of a UN Security Council mandate to use force once it decides to contribute with an EU mission. If the EU decides not to accept a UN Security Council mandate, the thesis argues that the EU is under the obligation not to undermine the success of a UN authorised military intervention, in the spirit of a loyalty obligation. Apart from analyzing the interaction of the EU and the international legal framework, the thesis also uses a speculative approach in order to examine the implications of silence in the context of the use of force.