World events since late 2013 have led some observers to conclude that the international security environment is undergoing a shift from the familiar post-Cold War era of the last 20-25 years, also sometimes known as the unipolar moment (with the United States as the unipolar power), to a new and different strategic situation that features, among other things, renewed great power competition and challenges to elements of the U.S.- led international order that has operated since World War II. A shift in the international security environment could have significant implications for U.S. defense plans and programs. This book discusses potential implications for defence and issues for Congress that arise with a shift in the international security environment. It also examines outcomes and key challenges of NATO's Wales Summit; priorities for 21st century defence; and discusses the 2014 Quadrennial Defense Review and defense strategy.