Queen Elizabeth Wanted Prince William and Prince Harry to Serve …

Queen Elizabeth expected grandsons Prince William and Prince Harry to "do their duty" by serving their country.

In the new ITV documentary The Real Crown: Inside the House of Windsor, former British Army officer General Sir Mike Jackson recalled a conversation he had with Queen Elizabeth about her grandsons.

"The chiefs have an audience with The Queen once or twice a year. You need to have done your homework," he recalled. "She's very wise -- I used to tell my staff, see if you can get the midday slot. Um, because after half an hour or 40 minutes, the Queen would ring a small bell and 'Time for a Sherry, I think, CGS."

"What goes on in those audiences and who says what to whom, remains for the two people involved," he continued. "And I will break the rule, uh, about not divulging what goes on on this one occasion when she was very clear -- she said, 'My grandsons have taken my shilling, therefore they must do their duty.' And that was that."

Jackson added that with Prince William as second in line to the throne at the time, the risk of him serving on the frontlines was deemed "too great." However, he said "the risk was acceptable" for Prince Harry.

Prince William, now 41, spent over seven years in full-time military service.

Following the completion of his time with the Household Cavalry, he undertook attachments to the Royal Air Force and the Royal Navy. He spent three years as a search and rescue helicopter pilot before retraining as a pilot with the East Anglian Air Ambulance.

Prince Harry, now 38, graduated from the Royal Military Academy Sandhurst in 2005 and served in the British Armed Forces for a decade. He rose to the rank of Captain, deploying twice to Afghanistan and completing elite training to fly Apache helicopters.

Prince Harry wrote extensively about his military experience in his memoir Spare, released earlier this year, saying that he wanted to leave the war in Afghanistan with his "conscience intact."

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In an exclusive interview with PEOPLE surrounding the release of his book, the Duke of Sussex addressed how he reconciles the ethical impacts of war.

"I don't know that you ever fully reconcile the painful elements of being at war.

This is something each soldier has to confront, and in the nearly two decades of working alongside service personnel and veterans, I've listened to their stories and have shared mine," Harry told PEOPLE.

"In these conversations, we often talk about the parts of our service that haunt us -- the lives lost, the lives taken.

But also the parts of our service that heal us and the lives we've saved," he added.

"It's a duty, a job and a service to our country -- and having done two tours of duty in Afghanistan for my country, I've done all I could to be the best soldier I was trained to be," Prince Harry said. "There's truly no right or wrong way to try and navigate these feelings, but I know from my own healing journey that silence has been the least effective remedy.

Expressing and detailing my experience is how I chose to deal with it, in the hopes it would help others."