Peter Rutland is a professor of government at Wesleyan University in Middletown, Connecticut. Philip Pomper is author of Lenin’s Brother: The Origins of the October Revolution.

Twenty years after the August 1991 coup that led to the collapse of the Soviet Union, it is worth revisiting the puzzle of the Soviet Union’s abrupt demise. Which individual more than any other should be held responsible for the Soviet collapse? The usual answers would be Soviet leader Mikhail Gorbachev (for liberals) or U.S. President Ronald Reagan (for conservatives). But in reality, only one figure deserves the credit: Josef Stalin.

Stalin is often portrayed as an evil strategic genius who took advantage of the weakness of the West and the presence of the Red Army in Berlin in 1945 to expand the Soviet empire deep into Europe.

In reality, Stalin’s projection of Soviet power into Central Europe was a strategic blunder that ultimately doomed the Soviet state. Stalin fully accepted Vladimir Lenin’s argument that imperialism was “the highest stage of capitalism.” This meant that as long as capitalism existed, it would try to expand through imperialist wars and territorial conquest. To protect the Soviet Union from such an attack, Stalin decided to maintain his giant armies in peacetime and to invest in securing a huge swathe of real estate in Eastern Europe as a buffer zone against future assaults.

But Stalin’s strategic thinking was terribly out of date. There would be no imperialist attack in the decades after 1945. The deployment of intercontinental ballistic missiles with nuclear warheads made war between the superpowers unthinkable. Moreover, the imperialist mind-set had destroyed itself in the successive bloodbaths of World War I and World War II. In the decade after 1945, European colonial empires were in the process of disintegration, and the United States itself was not interested in building an empire or starting any new massive land wars.

Thus, Stalin was protecting himself against a military threat that no longer existed and was turning the Soviet Union into a multinational empire at the very moment when the practice of empire-building became an anachronism and nationalism was growing in strength...