This is the third revised edition of a well-established and authoritative book with a comprehensive overview of research on interstate war. For this edition, the editors have added a chapter that provides a stimulating discussion of Russia’s full-scale invasion of Ukraine in 2022, drawing on patterns discussed in the earlier thematic chapters. For example, territorial disputes between neighbours are common and often escalate into wars, whereas nuclear weapons and Cold War norms may prevent further escalation. As shown in the chapter by Owsiak, Diehl & Goertz on the conditions for positive peace, some states can peacefully handle even complex issues, such as historically disputed territories and military alliances, without resorting to war or nuclear deterrence. Factors such as stable joint democracy and integration explain how war can become unthinkable between former archenemies like Sweden-Denmark and France-Germany. In this perspective, Russia’s authoritarian regime remains the critical obstacle to real peace in Europe. The authors stress that this war represents the most common type of interstate war – a territorial war of conquest between two neighbours. However, based on patterns observed in the period since World War II, the war is unlikely to end in a peace agreement leading to international recognition of Russia’s conquests. Aside from cases related to state formation in the context of decolonization, there is no clear case since the 1940s of a successful interstate war of conquest leading to substantial territorial gains recognized by more than a few states. Allowing Russia to annex parts or all of Ukraine, as some pundits advocate at the time of writing, would thus set a dangerous new precedent.