Prince Charles and Princess Diana‘s “miserable” 1992 tour of South Korea has been viewed for years as the trip that put the nail in the coffin of their marriage. But in a new episode of the Daily Mail’s Queens, Kings and Dastardly Things podcast, royal biographer Robert Hardman shared what it was really like on that fateful royal tour—and how what seems like a relatively minor incident was what actually made them separate.

Hardman, who was a new royal correspondent at the time for the Daily Telegraph, said that South Korea was his first-ever royal tour and an “eye-opening experience.” He continued, “The thing that sticks in my mind was that, on the one hand, you had the royal soap opera. It was clear it wasn’t an easy time for them.”

He added, “But I remember thinking, ‘Well done both of you,’ because this is pretty miserable and they were putting their best foot forward because you’re doing this for the country.”

“There was this subtext of unhappiness,” he shared, referencing a famous photo of the couple “looking particularly downcast.” Their press nickname of “The Glums” stuck throughout the South Korea trip, which ended up being their final joint royal tour.

However, Hardman stressed that the difficult tour wasn’t what finally pushed Charles and Diana to separate. “That wasn’t what brought things to a final halt. It was actually, as ever in these things, it’s often the sort of small thing that finally is the straw that breaks the camel’s back,” he said.

The royal biographer shared that Charles and Diana were to show a “united front” during “a shooting weekend up at Sandringham.”

Hardman explained, “There were going to be lots of other families there and lots of other children there, they were all going to kind of get together with all these other families and Diana, she said, ‘No, I’m not going. I’m taking the boys elsewhere.'”

Prince Charles didn’t react well to his wife declining the event, with Hardman stating that the now-King “said, ‘Well hang on, I’ve got this whole weekend organized.’ And she just said ‘No, I’m not doing it.'”

According to Hardman, it was the final straw for the couple. He continued that the Sandringham incident “was the moment when Charles and Diana just thought, ‘Okay, this can’t carry on.’ That was when the separation was decided.”

Their official separation was announced on December 9, 1992, and the couple went on to finalize their divorce in 1996.