This book presents a fresh perspective on the geopolitical landscape surrounding NATOs enlargement, aptly termed "expansion by invitation." Looking beyond conventional narratives, it unravels the multifaceted outcomes of this process, extending beyond the initial optimism. While acknowledging the merits of NATOs expansion, the book scrutinizes its contentious side effects, notably the escalating militarization of the Baltic and Black Sea regions, heightened military expenditure, shifts toward offensive doctrines, and the surge in military exercises and incidents between NATO and Russia. In a world witnessing a renewed bipolarization akin to the Cold War era, the book underscores the need for NATO and Russia to de-escalate tensions and strive towards a transition from contemporary negative peace to a more constructive, positive peace. Devoid of bias, this book refrains from condemning the expansion process but rather calls for an acknowledgment of its consequences. It is an indispensable resource for academics seeking a nuanced understanding of NATOs expansion, challenging prevailing terminologies and offering insights into the complex dynamics of international security in Europe.