Throughout the post-war period, transatlantic security relations have been characterised by a high degree of continuity. As a result of the perceived Soviet threat, the United States decided to remain in Europe and, through the establishment of the North Atlantic Treaty Organisation (NATO) in 1949, it became involved in European security on a permanent basis. Moreover, backed by a huge nuclear arsenal and massive conventional forces, it became the most important guarantor of Western European security and the key player in the Alliance.