Russia's invasion of Ukraine seems to many like a throwback to another age, rattling Europe with memories of past horrors. But since the end of the Second World War there has not been a single day without armed conflict somewhere in the world. Drawing on the great political philosophers, from Plato to Marx, via Machiavelli and Hobbes, Frédéric Gros attempts to answer the age-old questions regarding humanity's propensity to wage war: What is a just war? What moral constraints operate on the combatants? Does the state make war or does war make the state? Finally, after exploring the meaning and the spectre of total war, he tackles the ultimate question: Why war?