A century ago a citizen army stopped a Soviet takeover of Europe. Communism arose in a time of pandemic and war. In 1918, the Bolshevik party led by V. I. Lenin seized control of Russia amid a world numbed by the slaughter of a world war and the seemingly unstoppable spread of influenza. Promising a utopia of equality, the communist message proved seductive to many in Europe and America. In the summer of 1920, having defeated his most serious internal foes, Lenin decided to launch a full-scale invasion of Europe to spread communism all across the continent. Soviet communism proved...