Sometimes an image has a troubling, haunting quality that — to use a modern formulation — is hard to unsee.

One of these has pursued me now for about three weeks. It just pops up, unbidden, throughout the day, reviving a sense of despair about a segment of western youth that I hope is smaller than social media make it seem. The image belongs to a YouTube video of a typically oafish anti-Trump demonstration that took place in London during the president’s recent visit to the United Kingdom.

An older Brit, not quite yet in “elderly” territory, approaches the protesters, whose signs are giving the letter “F” its usual workout. He is wearing a MAGA hat and engages some of the protesters in debate. Initially, it seems to go well, with what appears to be an exchange of good-natured banter with the anti-Trump Brits.

Then it falls apart.

A group of protesters I might charitably describe as grubby, perhaps concerned that debate might confer respect upon a MAGA hat wearer, surrounds him and screams into his face, rapidly, loudly and repeatedly: “Nazi scum!” “Nazi scum!” His hat is knocked to the ground. Among the screamers is the young woman who constitutes my haunting image. Somewhere along life’s path she learned, or decided, that it is acceptable to get within six inches of someone’s face while screaming insults. Her face is contorted with fury. She smiles piggishly when someone throws a milkshake at the Trumper, and with that she disappears from the video but not from my head.

Two things:

First, when and how did it become acceptable to behave so atrociously toward anyone, let alone a senior citizen? Second, how did ignorance of what it meant to be a partisan of the National Socialist German Workers Party become so widespread? It is astounding that youngsters are so ignorant of the Nazi era that they should compare a constitutionally chained president of the United States with a dictator who exercised absolute and unchallenged power on his way to murdering millions of men, women and children.

It is useful, now and then, to refresh one’s consciousness of just how evil National Socialism was. It is surprisingly easy to forget.

I’m currently about halfway through Michael Jones’ 2008 book, “Leningrad: State of Siege,” which tells the story of Adolf Hitler’s decision to starve to death the more than 2 million inhabitants of the Russian city of Leningrad — formerly, and now once again, St. Petersburg. It is a searing account of weeping mothers desperate to feed their starving children, of an emaciated eight-year-old boy wandering the streets, insanely chanting the words, “Big piece of bread.” Furniture glue is scraped and boiled in a desperate effort to create some sort of soup.

About a million people died during the two-year siege.

The “Nazi” insult should be retired, so unmatched is the evil it represents.

It is a scandal upon modern youth that it is so lightly and ignorantly deployed.

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