The Duke of York said he returned from the Falklands War "a changed man" in a piece of writing posted to his ex-wife's Instagram account.
Prince Andrew, who reached a multimillion-pound out-of-court settlement in a civil sexual assault case a few weeks ago, wrote more than 700 words about his experience in the Falklands.
The Queen's second-eldest son flew missions as a Sea King helicopter pilot during the conflict.
Prince Andrew's reflection appeared in three posts - which were removed after about two hours - on the Instagram account of Sarah, Duchess of York.
Beneath the last post, it said it was "written by HRH The Duke of York" before the "HRH" was deleted.
The Queen stripped Andrew of his honorary military roles in January and he gave up his HRH style in a dramatic fallout from his civil sex case.
At the time, a royal source said Andrew, who was born a HRH, will not use it in any official capacity.
Virginia Giuffre was suing Andrew for allegedly sexually assaulting her when she was 17 and trafficked by convicted paedophile Jeffrey Epstein.
In the first Instagram post, Sarah wrote: "I asked Andrew this morning for his reflections on the anniversary of his sailing from Portsmouth to the Falkland Islands 40 years ago."
Andrew's account begins: "As I sit here at my desk on this cold crisp spring morning thinking back to April 1982 I've tried to think what was going through my mind as we sailed out of Portsmouth lining the flight deck of HMS INVINCIBLE."
The 62-year-old concluded: "So whilst I think back to a day when a young man went to war, full of bravado, I returned a changed man.
"I put away childish things and false bravado and returned a man full in the knowledge of human frailty and suffering.
"My reflection makes me think even harder and pray even more fervently for those in conflict today, for those family's (sic) torn apart by the horrors they have witnessed.
"And, i'm (sic) afraid to say, that the historical perspective my short war has taught me is this - war is failure to keep peace; war is failure of human judgement; war is failure to recognise we need to seek permission to understand another persons perspective or reality, whether or not we agree or disagree with that perspective or reality."
Prince Andrew also recalled being shot at, writing: "I was flying and saw a chaff shell fired from one of our ships that passed not that far in front of us.
"For a moment it was on a steady bearing before it began to cross to our left.
"The terror that that was going to be that, just for a moment, has had a lasting and permanent effect on me."
The Duke of York has spoken about being shot at before - in his infamous Newsnight interview, given to defend himself against Ms Guiffre's accusations and to explain his friendship with the late Epstein.
Addressing a claim he was sweating heavily during an alleged night out with Ms Guiffre, Andrew told Emily Maitlis in 2019: "I didn't sweat at the time because I had suffered what I would describe as an overdose of adrenaline in the Falklands War when I was shot at and I simply...it was almost impossible for me to sweat."
Andrew was the first member of the royal family to have an official Twitter account under his own name, though that account was deleted in January.
His reflections on Sarah's Instagram account came on the 40th anniversary of the Falklands War.
On April 2, 1982, Argentine forces invaded the islands, which had been in British hands since the 19th century, sparking the sending of a Royal Navy task force south to recapture them.
The resulting naval and land campaign led to the recapture of the islands on June 14 at the cost of 255 British lives. About 650 Argentines died.
