Prince William and Prince Harry knew early on that they were heading down very different paths

Prince William and Prince Harry were aware from an early age that their lives would take very different trajectories, shaped by the tumultuous environment of their parents’ fractured marriage. In historian Robert Lacey’s new book, Battle of Brothers: The Inside Story of a Family in Tumult, which features insights in this week’s issue of PEOPLE, the impact of their upbringing is laid bare.

Lacey writes, “From the ages of 4 and 2, William and Harry grew up in the care of two parents who were not sharing the same bed, who were more inclined to talk to the press than to each other, and who were also engaging in patterns of systematic deceit.” The ongoing affair of their father, Prince Charles, with Camilla, now his wife, added further stress to their childhood.

As the once “rambunctious” William grew more reflective and introverted upon entering boarding school, Harry’s personality also evolved. Ken Wharfe, the boys’ bodyguard in the late 1980s, recalls a revealing moment during a family trip from London to Highgrove. In the backseat, an argument erupted between the brothers, with Harry declaring, “You’ll be king one day. I won’t. So I can do what I want.” A stunned Princess Diana responded, “Where the hell did he get that from?”

Lacey emphasizes that both brothers have been deeply affected by their upbringing, reacting in different ways. He explains, “Both brothers have been damaged by their upbringings; both have reacted by finding different solutions.” The pain and trauma woven into their story trace back to their earliest days.

Though they were raised to be close and protective of one another, tensions escalated as William prepared for his future role as monarch, while Harry struggled to find his own path. Now living in different countries—Harry and Meghan Markle having relocated to California after stepping back from their senior royal duties—the stakes have never been higher.

Lacey warns of “potential tragedy” if the brothers cannot reconcile, comparing their situation to the abdication of Edward VIII in 1936 and the death of Princess Diana in 1997. “It’s of that scale. And it’s a challenge that’s yet to be resolved,” he concludes.