Economists as diverse in approach as Lord Keynes and Milton Friedman have analyzed the causes of the Great Depression, and their answers have ranged from underconsumption to failings in monetary policy. In Part 1 of "The Great Depression," Christian Saint-Etienne compares these theories with economic statistics for the interwar period and adds a further explanation: a collapse in international trade initiated by the Hawley-Smoot Tariff in the United...