A European Army featured at the very beginning of the European integration process. In the early 1950s the ‘Plan Pleven’ proposed to establish a European Defence Community comprising inter alia of an integrated European Defence Force. However, the plan failed and the notion of a European army disappeared from the European agenda for a long time. While the creation of a European army is controversial and not very likely in the short term, the (Common) European Security and Defence Policy developed since the late 1990s might well lead to a permanent European military force in the medium or long term. However, so far EU military missions (in Bosnia, the DRC, Mali, or the Horn of Africa), while based on a permanent intergovernmental framework and EU military bodies, have been conducted by forces made up of national Member State forces formed on an ad hoc basis. The paper will examine the legal and policy arguments for a European army and discuss how the existing legal framework under the Treaty of Lisbon would need to be reformed to permit the establishment of such an entity.