Across the 1930s but especially 1937 and 1938, Stalin ordered the arrests of some 1.6 million party officials, military officers, intelligence agents and others on trumped-up charges of betraying the nation, a stunning display of ruthlessness that gutted Soviet leadership circles at a time of mounting threats from abroad. Some victims were convicted in dramatic show trials. Far more were murdered quietly, often after being tortured into confessing their supposed crimes.

Stalin was motivated in part, Kotkin asserts, by his determination to break the will of critics and rivals. In this sense, the terror “constituted a form of rule, a matter of statecraft” that sprang readily from a mind steeped in paranoia but capable of impeccable self-control. Kotkin also suggests another, more intriguing explanation: Stalin used the purges to open opportunities for younger, well-educated functionaries he judged better able to advance the nation’s industrial development.

Kotkin’s most striking contribution, though, is to probe reasons Stalin encountered little opposition as he wrought mayhem on his nation. Careerism and bureaucratic incentives in the Soviet Union’s formidable apparatus of repression had something to do with it, Kotkin writes, but so too did the party’s monopoly on information and the public’s receptiveness to wild claims about the danger of subversion from within. Stalinism was, in this way, as much enabled from below as imposed from above.

In the third section of the book, Kotkin turns to geopolitics, which increasingly preoccupied Stalin as Nazi Germany and militarist Japan upended the global status quo in the late 1930s. Well aware of Soviet military weakness yet eager to expand his nation’s borders and enhance his power, Stalin faced difficult decisions: Should he cut deals with the aggressors, despite their rabid anti-Communism, or throw in his lot with Britain? The answers he reached from 1939 to 1941 reveal Stalin as both consummate opportunist and narrow-minded ideologue.

The opportunist shone through in 1939, when Stalin signed his notorious nonaggression pact with Germany in return for territorial gains in eastern Poland and the Baltic nations. But Stalin’s ideological commitments resurfaced. Certain that arch-capitalist Britain was his country’s foremost enemy, he clung to his strange-bedfellows partnership with Germany long after Hitler had abandoned the arrangement. The Soviet Union was ill prepared when Nazi armies invaded in June 1941.

Kotkin’s account of this complex diplomatic dance disappoints only because it seems to turn him away from the more intimate, often chilling detail that he sprinkles into earlier sections of the book. Kotkin’s eye for revealing minutiae — Stalin’s failure to attend his mother’s funeral and penchant for doodling pictures of wolves during meetings, for example — does much to bring the dictator alive in the years before 1938. One is left to wonder about Stalin’s inner world as war grew close.