• Historian Robert Lacey recently released his book on Princes William and Harry, "Battle of Brothers"
  • He believes Prince William's alleged warning to Prince Harry about Meghan Markle turned out to be correct
  • The royal author said the Duke of Cambridge has done more for "woke" causes than the Sussexes

Prince William reportedly raised concerns when Prince Harry started dating Meghan Markle, and a royal author believes that the Duke of Cambridge's warning to his younger brother turned out to be right.

In an interview with True Royalty TV's "The Royal Beat," Robert Lacey, author of "Battle of Brothers" talked about Prince William's relationship with Prince Harry and Markle. While Lacey said in his own book that the tension between the brothers predates Markle's arrival, the Sussex biography "Finding Freedom" said that they had a falling out when Prince William advised Prince Harry not to rush in his relationship with the former actress.

For the historian, Prince William's concerns were valid.

"I think when William sat down with Harry and said there could be problems, I think events have proved him right," Lacey, who works as a historian on Netflix's "The Crown," said.

"Meghan acknowledged that it's risky for her family to get involved in controversial issues," he continued. "Maybe she's simply talking about the physical risk that exists in America for anybody, because of the way politics has become so volatile and violent there... but I think that she has come to realize how [some of her] speeches risk her husband's relationship with the family and perhaps she may pedal back on some of these more extreme positions."

Lacey also compared Prince William’s work with that of the Sussexes.

Markle has already made six digital appearances this month alone, including Fortune's Most Powerful Women Next Gen Virtual Summit. She and Prince Harry also sparked controversy last month when they appeared in a Time 100 video and urged Americans to vote and "reject hate speech, misinformation and online negativity."

For Lacey, the Duke and Duchess of Sussex were "too preachy" compared to Prince William, who recently launched his Earthshot prize that aims to find solutions to repair the planet by 2030.

"People have said the Windsors don't do 'woke'. They don't know how to handle progressive causes," the historian said. "However, I think William does it rather well. He does it better than Meghan and Harry. This couple, in a way, are too extreme."

Lacey also suggested that the alleged rift between the brothers may worsen if Prince Harry's role as head of the Royal Marines will be given to Prince William.

"I think that will mark the rift," he said. "I think that they have kept that position open for him [Harry] if he wants to come back into the family and if they are going to give it to William, I think that is the end of it [the relationship between the boys]."

Prince Harry last year revealed that he had grown apart from his older brother William
Prince Harry last year revealed that he had grown apart from his older brother William AFP / Tolga AKMEN