There is a widely circulated quote about totalitarianism that reads:
When people are atomised, a movement or a strongman arises and he offers a story or an ideology which claims to explain everything, why people are unhappy.
It's widely misattributed, and this page exists to document the correct author.
The short answer but sadly not simple answer is that Helen Nianias is paraphrasing Robert Eaglestone who is himself summarizing Hannah Arendt.
In “What Hannah Arendt can teach us about totalitarianism” Helen Nianias writes:
Arendt believed that a society in which individuals are disconnected from each other is most vulnerable to totalitarian leaders.
“When people are atomized, a movement or a strongman arises and he offers a story or an ideology which claims to explain everything, why people are unhappy,” explained In Our Time’s guest Robert Eaglestone. “This story becomes more and more powerful. You can’t argue with people who become Nazis or Stalinists because there’s only one way to think.”
(Emphasis mine.)
That's the widely quoted line. However, I believe that is a paraphrase, not an exact quote. When Professor Eaglestone was interviewed for In Our Time, he said:
Eaglestone: Totalitarism arises out of— when people are disconnected from each other, when they're sort of atomized, and when social bonds aren't as strong as they have been, and a movement or a strongman arises, and he offers a story, an ideology, which claims to explain everything, why people are unhappy, to its adherents, and this story becomes more and more powerful. It means that you can't argue with people who have become Nazis or Stalinists, and—
Interviewer: Because there's only one way to think?
Eaglestone: Because there's only one way to think.
(Transcription and emphasis my own.)
Now, the context of the discussion is clearly Hannah Arendt. So, is Professor Eaglestone quoting or paraphrasing her? As best I can tell, no. I asked Eaglestone and he said he is summarizing part 3 of Arendt's The Origins of Totalitarianism, in particular chapter 10.
As of January 2022, I have not yet had the opportunity to give Arendt's Origins the attention it deserves. I've skimmed parts and searched it in an attempt to find the quote, and as far I can tell, Arendt did not summarize herself as the widely misattributed quote.
Nianias, Hellen, “What Hannah Arendt can teach us about totalitarianism”, bbc.co.uk. Probably around February 2017 based on earliest available Wayback Machine archive. Wayback Machine backup.
Bragg, Melvyn (Presenter), “Hannah Arendt”, In Our Time, 2017 (probably February 2, 2017). Official episode summary, includes download link. Wayback Machine backup. Direct link to MP3 of the episode. Wayback Machine backup of MP3. The relevant quote is at approximately 11:30 into the audio.
Arendt, Hannah, The Origins of Totalitarianism, 1951.