Had any of these events foiled Lenin, our own times would be radically different. Without Lenin there would have been no Hitler. Hitler owed much of his rise to the support of conservative elites who feared a Bolshevik revolution on German soil and who believed that he alone could defeat Marxism. And the rest of his radical program was likewise justified by the threat of Leninist revolution. His anti-Semitism, his anti-Slavic plan for Lebensraum and above all the invasion of the Soviet Union in 1941 were supported by the elites and the people because of the fear of what the Nazis called “Judeo-Bolshevism.”

Without the Russian Revolution of 1917, Hitler would likely have ended up painting postcards in one of the same flophouses where he started. No Lenin, no Hitler — and the 20th century becomes unimaginable. Indeed, the very geography of our imagination becomes unimaginable.

The East would look as different as the West. Mao, who received huge amounts of Soviet aid in the 1940s, would not have conquered China, which might still be ruled by the family of Chiang Kai-shek. The inspirations that illuminated the mountains of Cuba and the jungles of Vietnam would never have been. Kim Jong-un, pantomimic pastiche of Stalin, would not exist. There would have been no Cold War. The tournaments of power would likely have been just as vicious — just differently vicious.

The Russian Revolution mobilized a popular passion across the world based on Marxism-Leninism, fueled by messianic zeal. It was, perhaps, after the three Abrahamic religions, the greatest millenarian rapture of human history.

That virtuous idealism justified any monstrosity. The Bolsheviks admired the cleansing purges of Robespierre’s Reign of Terror: “A revolution without firing squads is meaningless,” Lenin said. The Bolsheviks created the first professional revolutionaries, the first total police state, the first modern mass-mobilization on behalf of class war against counterrevolution. Bolshevism was a mind-set, an idiosyncratic culture with an intolerant paranoid wordview obsessed with abstruse Marxist ideology. Their zeal justified the mass killings of all enemies, real and potential, not just by Lenin or Stalin but also Mao, Pol Pot in Cambodia, Mengistu Haile Mariam in Ethiopia. It also gave birth to slave labor camps, economic catastrophe and untold psychological damage. (These events are now so long ago that the horrors have been blurred and history forgotten; a glamorous glow of power and idealism lingers to intoxicate young voters disenchanted with the bland dithering of liberal capitalism.)

And then there is Russia, the successor to the Soviet Union. President Vladimir Putin’s power is enforced by his fellow former K.G.B. officers, the heirs of Lenin and Stalin’s secret police. Mr. Putin and his regime have adopted the Leninist tactics of “konspiratsia” and “dezinformatsiya,” which have turned out to be ideally suited to today’s technologies. Americans may have invented the internet, but they saw it (decadently) as a means of making money or (naïvely) as a magical click to freedom. The Russians, bred on Leninist cynicism, harnessed it to undermine American democracy.