M.E. Sarotte, a Johns Hopkins historian who specializes in US-European relations, examines how promises made during German reunification about NATO expansion became a source of lasting conflict with Russia. Drawing on recently declassified documents and interviews with key participants, Sarotte shows that American and West German leaders did suggest to Soviet officials that NATO would not expand eastward, even if these suggestions stopped short of formal commitments. She traces how these understandings were abandoned in the 1990s as NATO expanded to include former Warsaw Pact countries and eventually former Soviet republics. Sarotte examines the complex diplomacy that produced these outcomes, showing how different actors had different understandings of what had been promised. She explores how Russia experienced NATO expansion as a betrayal of trust that contributed to the deterioration of post-Cold War relations. The book does not excuse Russian aggression but provides essential context for understanding why Russian leaders view Western actions as hostile. Sarotte writes with the precision of a scholar who has mastered vast documentary evidence while maintaining narrative momentum. Essential reading for understanding the origins of current tensions between Russia and the West, the book illuminates three decades of diplomacy that shaped the world we now inhabit.