This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book offers an assessment of the collective defence dimension of the Warsaw Summit. It presents an introduction and background to the key decisions taken in Wales and Warsaw, and assesses some main implications of NATO's adaptation measures. The book examines how not only capacities but also doctrine and culture are important dimensions of a credible deterrence posture. It examines NATO's decision to enhance its forward presence in Eastern Europe. The book focuses on how NATO has elevated its nuclear deterrence profile in response to Russian nuclear challenges. It describes the geographical split in NATO that has become more discernible since the Wales Summit in 2014. The book also examines questions concerning the status of Finland and Sweden as nonaligned partners of NATO, and what impact this has on security in the Baltic Sea region.