"Both brothers have been damaged by their upbringings," says historian Robert Lacey

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Prince Harry and Prince William's fractured relationship began long before their wives joined the royal family.

When Harry, 36, and wife Meghan Markle separated their home and office from William, 38, and Kate Middleton in 2019, the siblings' rift became clear to the public. However, historian Robert Lacey's new book Battle of Brothers: The Inside Story of a Family in Tumult, excerpted in this week's issue of PEOPLE, explores the deep-rooted nature of the princes' schism dating back to their parents' turbulent marriage.

"Both brothers have been damaged by their upbringings; both have reacted by finding different solutions," says the author, who is also the historical consultant for Netflix’s The Crown. "There is so much pain and trauma in this story, going right back to the beginning."

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The Prince and Princess of Wales with sons Prince William, right, and Prince Harry
Princess Diana, Prince Harry, Prince Charles and Prince William
| Credit: PA Images via Getty Images
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Prince Harry and Prince William

Lacey says the brothers "were raised to be close and protective of each other," but tensions grew as William was prepped for his role as future monarch while Harry was left without a defined role of his own. And when Prince Harry found a partner in Meghan, William cautioned that the romance was moving too fast.

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"Fundamental to the whole saga is the clash of love versus duty," explains Lacey, adding that the brothers — now residing in different countries after Prince Harry and Meghan relocated to California after stepping down from their senior roles within the royal family — face "potential tragedy" if they can't reconcile.

Prince William and Prince Harry
Prince William and Prince Harry
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"This compares with Edward VIII’s abdication in 1936 and the death of Princess Diana in 1997," says Lacey. "It’s of that scale. And it's a challenge that’s yet to be resolved."