World War II began 80 years ago this Sunday after Nazi Germany and the Soviet Union signed a “nonaggression” pact that was, in fact, a mutual aggression pact. Adolf Hitler invaded Poland on Sept. 1, 1939. Russia’s invasion of Poland, no less murderous, followed two weeks later.

On Nov. 3 of that year, Joseph Goebbels, the propaganda minister, gave Hitler a report of his trip to Poland. “Above all, my description of the Jewish problem gets [Hitler’s] full approval,” he wrote in his diary. “The Jew is a waste product. It is a clinical issue more than a social one.”

For several years many commentators, including me, have written about the parallels between the prewar era and the present.

There’s the rise of dictatorial regimes intent on avenging past geopolitical humiliations and redrawing borders: Germany, Italy, Japan and Russia then; China, Iran and Russia now.

There’s the unwillingness of status quo powers to coordinate their actions, confront dictatorships, stamp out regional wars and rise to global challenges. The League of Nations then; the G7 now.

There’s the upsurge of nativist rancor, protectionist barriers and every-nation-for-itself policies, along with deep doubts about the viability of liberal democracy and the international order. Father Coughlin and the America Firsters then; Donald Trump and the America Firsters now.

All that, plus three crucial factors: new forms of mass communication, the rhetoric of dehumanization and the politics of absolute good versus absolute evil.

The (relatively) new technology of the 1930s was the radio. “It is the miracle of radio that it welds 60,000,000 Germans into a single crowd, to be played upon by a single voice,” The Times reported in 1936. This was by design. Among Goebbels’ first efforts after the Nazis came to power was to produce and distribute a cheap radio — the Volksempfänger, or people’s receiver — that could bring the Führer’s voice and message into every home.

The radio made possible an unmediated, seemingly personal relationship between leader and subject. It cut out the information brokers — reporters, editors, spokesmen, pundits and so on — on whom previous generations of leaders had been forced to rely. It turned a nation into an audience and politics into a theater where emotion mattered much more than sense. In “The Nightmare Years,” the CBS correspondent William Shirer recalled being struck by the complete disconnect between the insanity of Hitler’s language and the spellbinding quality of his delivery.

Radio then, like Twitter today, was the technology of the id; a channel that could concentrate political fury at a time when there was plenty to go around.

It was also a time when ideology dictated that fury be directed at entire classes of people. The decade began with Soviet propaganda cheering Stalin’s announcement of “the liquidation of kulaks as a class” — a reference to millions of Ukrainian peasants who would die of forced starvation in the Holodomor.

The political mind-set that turned human beings into categories, classes and races also turned them into rodents, insects and garbage. “Anti-Semitism is exactly the same as delousing,” Heinrich Himmler would claim in 1943. “Getting rid of lice is not a matter of ideology. It is a matter of cleanliness.” Watching Warsaw’s Jewish ghetto burn that year, a Polish anti-Semite was overheard saying: “The bedbugs are on fire. The Germans are doing a great job.”

Today, the rhetoric of infestation is back. In the U.S., Trump uses it to describe Latin American migrants. In Europe, Jaroslaw Kaczynski, chairman of Poland’s ruling Law and Justice Party, warned in 2015 that migrants carried “all sorts of parasites and protozoa,” which, “while not dangerous in the organisms of these people, could be dangerous here.”

More of this talk will surely follow, and not just from the right. The American left has become especially promiscuous when it comes to speaking pejoratively about entire categories of disfavored people.

None of this would be possible without the third factor: the conviction that an opponent embodies an irredeemable evil, and that his destruction is therefore an act of indubitable good. That spirit of certitude that dominated the politics of the 1930s is not so distant from us today. The unpopular political figures of our day are the people who seem to convey less than 100% true belief: the moderate conservative, the skeptical liberal, the centrist wobbler.

This 80th anniversary of World War II is an opportunity to reconsider how the world reached that dark defile, in which some 70 million people died. An opportunity, too, to remember the words of the American judge Learned Hand, on how free and civilized people can come back from the brink.

“The spirit of liberty,” he said, “is the spirit which is not too sure that it is right.”

Bret Stephens | The New York Times, (Tony Cenicola/The New York Times)

Bret Stephens is an Op-Ed columnist for The New York Times.