The Russian decision to mobilize in July 1914 may have been the single most catastrophic choice of the modern era. Some articulate, thoughtful figures around the Tsar understood Russia's fragility, and yet they were shouted down by those who were convinced that, despite Germany's patent military superiority, Russian greatness required decisive action. Russia's rulers thought they were acting to secure their future, but in fact - after millions of deaths and two revolutions - they were consigning their entire class to death or exile and their country to a uniquely terrible generations-long experiment under a very different regime.