On June 11, 1937, a closed military court ordered the execution of a group of the Soviet Union's most talented and experienced army officers, including Marshal Mikhail Tukhachevskii; all were charged with participating in a Nazi plot to overthrow the regime of Joseph Stalin. There followed a massive military purge, from the officer corps through the rank-and-file, that many consider a major factor in the Red Army's dismal performance in confronting the German invasion of June 1941. Why take such action on the eve of a major war? The most common theory has Stalin fabricating a "military...