When Empires Collide is a history of the famous encounter between the French philosopher Denis Diderot and his patron, Empress Catherine II of Russia, in 1773. The book begins many years earlier and traces the life of Diderot and Catherine in alternating chapters, painting a vivid and complex portrait of eighteenth-century Europe where new Enlightenment thinking co-existed with old monarchical systems. Robert Zaretsky has written an intellectual and political history of the time by spotlighting the exchange of ideas between a philosopher who reflected on the nature of power and a ruler who excercised it. In the autumn of 1773, taking up residence in Saint Petersburg at Catherine's invitation, the two met every third day for Catherine's instruction in various philosophical and political subjects. Zaretsky describes the scene: "For each 'lesson,' he prepared a series of notes on a particular theme, from which he would read at the start of the session. Having thus introduced the theme, Diderot then engaged Catherine in conversation. This made for a stunning tableau: the conversations were freewheeling, free of the protocol that reigned elsewhere in the palace. Catherine frequently knitted or embroidered during these sessions, while Diderot, caught up in the excitement of an idea, would whirl his hands to emphasize points, often reaching out and grabbing Catherine's leg or arm. Zaretsky pieces together their conversations from letters to each other and to other correspondents, as well as from Diderot's (still untranslated) memoirs. The influence seems to run in both directions; however, as the author concludes, this extraordinary friendship reveals two individuals aware of the power of ideas, but who have very different understandings of the use of ideas.--