As the stock market crash of 1929 plunged the world into turmoil, two men emerged with competing claims on how to restore balance to economies gone awry. John Maynard Keynes, the mercurial Cambridge economist, believed that government had a duty to spend when others would not. He met his opposite in a little-known Austrian economics professor, Freidrich Hayek, who considered attempts to intervene both pointless and potentially dangerous. The battle...