According to Lacey, William and Harry didn't always fit into the reputations they have now, namely, that William is thought of as the more serious of the two while Harry is thought to have a more rebellious nature.
"One of the objectives of the book is to go back into the past of these two boys, into their childhoods, to see the way in which their lives were shaped, cruelly really, by the breakdown of their parents' marriage and by the royal roles and distortion that was placed upon them," Lacey tells ET's Nischelle Turner. "You know, I describe how when they were born, it was William who was the rambunctious character and it was Harry who was the good little boy who sucked his thumb. Then William discovers that he's going to be king, he becomes much more serious and quiet between the age of six or seven. Harry goes the other way."
Lacey says that although the tension between the two brothers started well before Harry began his whirlwind romance with his now wife, Meghan Markle, her arrival did escalate it.
"I would say that there were stresses and strains there before and then with the advent of Meghan, that was what provoked the really big break," Lacey says. "William, from all the turmoil of their childhood, took strength from the idea of his duty and what lay ahead for him as king... Harry caught onto the importance of love. So, when he found Meghan, he went for that, and when William said to him, 'Look, perhaps you should slow down a bit,' Harry exploded. William then went to his uncle, Charles -- who is Diana's brother who had been a sort of godfather to the boys -- and asked him to intervene and suggest to Harry that he slow down. And Harry rejected William's model. He knew what he wanted, he loved this woman, and he was going to marry her so that was certainly the beginning of the rift."
According to Lacey, both brothers do have a temper, which may surprise those who think of William as being much more even-keeled.
"There are numerous occasions when in recent times the two brothers have lost their temper with each other -- it's been quite well known that Harry has a volatile temper," he says. "As I started writing the book, a lot of people close to the scene would say, 'Well, you know that William has a temper as well.' He would get angry with his father, Charles, when he didn't feel that Charles was doing the right thing as a future king. We know this because [Prince Charles' wife], Camilla, was shocked when she sometimes found William upgrading and insulting his father. She talked about it to family and friends -- that's how I found out about it."
Lacey also suggests that there have been times when William has been jealous of Harry.
"I talked to Ken Wolf, who was the bodyguard to Harry and William when they were little boys," he shares. "And he said how William was always sort of jealous of the lovable rogue reputation that Harry had and the affection that he generated. And there is certainly a school of thought that William and [his wife] Kate were not happy at the way in which these royal rock stars, Harry and Meghan, overshadowed them, and that they're quite happy now to have seen them out of the country."
"There is a school of thought that it suits William very well for Harry and Meghan to be the scapegoats living abroad," he also says.
Still, he notes that despite their current issues, the brothers were definitely close growing up.
"William always felt so protective towards his younger brother," he shares. "This is a very appealing side of William. He'd had the same impulse towards his mother as things got bad for her. His was the shoulder that she cried on. But there's a sense also of course that this robbed him of his youth and I think the responsibility he had for Harry also robbed him of the, you know, carefree nature of his childhood. That is why we are told he -- until last year -- he certainly hadn't told his own son, Prince George, that he was going to be king one day. William felt that that was something that'd been a burden too much for him as a child and he didn't want that for his own son."
Lacey points out that the coronavirus pandemic has had a negative impact on the brothers repairing their relationship.
"When [Harry and Meghan] left in March, they were committed to coming back in the summer for the queen's birthday and all the celebrations that go with that," he explains. "They were committed to coming back in the autumn for various military occasions that meant a lot to Harry and that has not been possible, so it may well be when and if life gets back to normal, the brothers can spend more time together and these fences could be mended."
"In Britain, we all hope there will be some sort of coming together that will require compromise on both sides or the alternative is that they decide ... that as brothers, we're going to go our own separate ways," he also says. "Next July the 1st, both brothers have promised to be in Kensington Palace gardens to celebrate the 16th anniversary of their mother's death with a statue they have commissioned to Diana. They are going to be there ... Prince Charles is going to be there, presumably, presumably Camila will not be there and the world will be looking at them and the world will want to know."
Though in Lacey's opinion, Harry and William are on separate journeys for good.
"I think we're gonna have to face the reality that these are two brothers that have drawn two different lessons from their troubled childhoods and they're going in different directions," he adds. "I think the best we can hope for is that they can maintain their fundamental love and respect for each other and remain in good terms for the future, but I don't think we're going to see Prince Harry and Meghan as working royals back in the family. That doesn't mean to say though they won't remain members of the family respected for their different values and different opinions."
Battle of Brothers: William and Harry - The Inside Story of a Family in Tumult hits shelves on Tuesday.