This book analyses the expansion of The North Atlantic Treaty Organization (NATO) into the post-Soviet space after the end of the Cold War. Based on an extensive analysis of the literature and government documents, including doctrines, statements and speeches by the most influential decision-makers and other actors, it sheds new light on the geopolitical and geostrategic context of the expansion of the military alliance, and assesses its impact on international security relations in Europe. The first chapter introduces readers to the neo-realist approach and develops the methodological basis of the book. The following chapters provide a historical overview of the causes and consequences of two waves of eastward NATO enlargement. Special attention is paid to the annexation of the Crimea and to Russian hybrid-asymmetric warfare. Finally, thirty years after the end of the Cold War, the book notes a disturbing return to militarization in international security relations. To counter this process, the author calls for a reduction of current international tensions and a new policy of détente.