As a teenager at the prestigious Eton College, Prince Harry made headlines around the world for his partying ways

With Prince William destined to be the future king, Prince Harry became the brother with the bad reputation.

Historian Robert Lacey explores the Duke of Cambridge and Duke of Sussex's fractured relationship in the new book Battle of Brothers: The Inside Story of a Family in Tumult, excerpted in this week's issue of PEOPLE. Lending to the rift were their differing roles — and Harry's positioning as the rebel.

As a teenager at the prestigious Eton College, Prince Harry made headlines around the world for his partying ways, including underage drinking and scraps with photographers. Harry visited a drug rehab clinic in 2002 to learn about the organization's work, but palace officials spun the visit as something Prince Charles had arranged for his younger son as a teachable moment.

"Dad might have emerged from the story smelling of roses, but it was Harry who was typecast as the 'Bad Boy of Buckingham Palace,' " writes Lacey.

Princes William and Harry
Prince Harry and Prince William in 2005
| Credit: Andrew Parsons/PA Images via Getty
Princes William and Harry
Prince William and Prince Harry in 2002
| Credit: Tim Graham Photo Library via Getty

In 2005, a 20-year-old Harry stepped out to a costume party with William — wearing a Nazi uniform including a swastika armband.

The incident "would be taken to represent his wild, foolish and totally unjudged side," Lacey says in the book.

"A fellow guest [snapped] a photograph, and a few days later, there was Harry parading on the front page of The Sun."

Prince Harry Eton Bedroom
Prince Harry at Eton
| Credit: Tim Graham Picture Library/Getty
prince harry
Prince Harry

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Prince Harry issued an apology for the "poor choice of costume," but what people didn't realize was Prince William's involvement. According to the book, "Harry chose his costume in conjunction with his elder brother, who laughed all the way back to Highgrove with the younger sibling he was supposed to be mentoring."

10/19/2020 people cover prince harry prince william

Lacey tells PEOPLE 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.

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

By Simon Perry and Steph Petit