For centuries liberal minded men have been horrified by the pain and waste of war. From Erasmus, who saw war above all as a product of stupidity, to the Marxists who see it as a matter of class conflict, they have produced social theories to account for its occurrence and have tried to devise means to end it.

Their prescriptions have been various. The central view of the Enlightenment was that wars would end when the ambitions of princes could be curbed by the sanity of ordinary men. At first the commercial classes seemed to be the new force that would produce this happy state, but by the...