From homeland defence to crisis intervention-this statement describes the conceptual transformation of the German army and NATO since 1990. The Crimean crisis in 2014 changed this situation, with homeland defence becoming a major concern again. However, the security policy environment, potential threats and the structure of the army and NATO have little in common with the traditional scenario of the Cold War. Entirely new challenges need to be dealt with-from new forms of conflict (asymmetrical and hybrid conflicts, cyber- and information warfare) to NATO's geography with its vulnerable periphery in the Baltic region. These challenges raise new legal questions, which are discussed in this conference volume.