History neither repeats itself for all historians, nor do all lawyers consider every case unique. When eminent Yale historian Odd Arne Westad warns that complacency could trigger a ‘Great Power war’ in ways similar to 1914 – and explains how that could happen in the main hotspots – his new book should be taken very seriously. As today’s world is ‘at least as messy as the world before World War I’ and the threat of nuclear destruction ‘no longer holds absolute deterrent power’, we should ‘prepare for how to avoid’ war. In a world where the security needs and ‘vital interests’ of Great Powers dominate, ‘smaller countries, as well as international organizations and agreements’ would be seen as ‘losers’. As Westad politely abstains from flagging humankind’s Great Power problem (Great Powers cause great problems), his book will not cause offence among the many diplomats and defence officials in Beijing, Brasília, London, New Delhi, Tokyo and Washington, DC who should read it. His emphasis on ‘effective deterrence’ and ‘Great Power compromise’ (‘not agreement, not convergence, not moral equivalence, but tentative deals for at least some of the issues’) – without dwelling on their UN Charter obligations – should make the book palatable also in Great Powers deeply uncertain or confused about their role in the world. Reconciling perceived national interests with ‘opportunities for compromise as they arise’ is key, while technology and other factors may further reduce the time available for diplomacy in crises. Westad adds the importance of ‘deep reasoning about the motives of all parties involved’ to weaken the impact of fear, resentment, misperceptions and miscalculations. His analysis of those motives contributes usefully to such reasoning.