Then-President Donald Trump and John Kelly.

Former President Donald Trump remarked in 2018 that "Hitler did a lot of good things," according to a new book, leaving his chief of staff John Kelly reeling.

Trump made the comments during a November 2018 tour of Europe taken to mark 100 years since then end of World War I, the Wall Street Journal reporter Michael Bender wrote in his upcoming book, "Frankly, We Did Win This Election," a copy of which was obtained by The Guardian.

Kelly was "stunned" by Trump's assertion, according to Bender, and when Kelly told him he was wrong Trump "was undeterred," praising Hitler's efforts to revive the German economy in the 1930s.

Kelly outright disagreed, saying Germans would have been "better off poor than subjected to the Nazi genocide," Bender wrote, according to The Guardian.

Kelly then told Trump: "You cannot ever say anything supportive of Adolf Hitler. You just can't," according to Bender.

Trump denied making the comment, Bender's excerpt said. Representatives for Trump did not immediately respond to Insider's request for comment.

Trump's comment about Hitler was prompted by Kelly giving Trump a brief history lesson about who fought on whose side during the Great War, Bender reported, according to The Guardian.

Kelly "reminded the president which countries were on which side during the conflict" and "connected the dots from the First World War to the Second World War and all of Hitler's atrocities," Bender writes, per The Guardian.

That 2018 trip to Europe was also controversial for Trump for another reason.

The Atlantic reported that ahead of a visit to the Aisne-Marne American Cemetery, a US military graveyard in France, Trump called the dead American soldiers resting there "losers."

On the same trip, Trump said the some 1,800 marines who died at Belleau Wood, France, in World War I were "suckers" for getting killed, The Atlantic reported.

Trump repeatedly denied making those remarks on the trip.

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