But he managed to keep hold of it, and for nearly three decades sat on it, an explosive real-life record which if published would have ended his career and even, he was warned, get him sent to jail.
Becoming a civilian defused the threats of the military establishment and the result was that that diary has finally been published as a book called The Reality of War.
And ironically John, who lives near Whitchurch, says the feedback has been surprisingly positive.
He talks of a British Army in which, at least in the unit in which he served, rank counts for more than expertise and knowledge, and ordinary soldiers often find themselves in effect treated, in his view, as officers' slaves.
Sent to the Gulf as part of Britain's response to Saddam Hussein's invasion of Kuwait, John started keeping the diary after a female American soldier sent him a blank book.
"I kept a diary mainly because it was something new, something different at the time and it gave me something to do as well," he said.
"As time went on I started realising there were lots of things going on and you think 'the public know none of this'.
"My perception of what war was like and what to expect was not what happened. Me and friends would chat and say 'people will never believe this, never believe what goes on.' I thought 'I'm going to write it all down'."
At first Army colleagues were a little scared about what he was doing, but as time went by they started to contribute and offer information, although he did not include it if it was not relevant or was their personal experience.
"At the end of the war my commanding officer wanted the book burnt," he said.
"I was threatened that if I was to publish they would go all bells and whistles and get me jailed for doing it."
His colleagues knew what was in it because he had allowed them to read it so they could factually verify it as he went along.
Clearly his revelations held an embarrassment factor.
"Our officers especially are a breed which are so unique. Unless you served in a cavalry regiment and came across them you would not believe what they were like."
One surprising war zone task involved laying a green carpet in one of the big tents in the desert – with any carpet left over being used on the floors of the Army vehicles.
An additional factor causing him to delay publication was that he wanted to avoid the possibility of giving encouragement to potential adversaries by making them think that the British Army was not as good as it liked to believe.
"I didn't want to put people in harm's way by putting a book out telling the truth about what we are like."
John, who is 57, served 37 years in the military, starting out in divisional reconnaissance and ending his career as an instructor on the Apache helicopter. His service included a tour in Afghanistan.
He moved to Shropshire when he worked at RAF Shawbury and although now a civilian, his work is still military-related.
"I teach in Europe – I teach military pilots how to operate in combat zones," he said.
Having served in Afghanistan, he prefers not to make his real name public – of course, his Army colleagues know his identity anyway – and has chosen the author's pseudonym John Wolf for a straightforward reason.
"I have always loved wolves. They are my favourite creature."
The book is available through Amazon and also has a dedicated website therealityofwar.com
He said: "I thought some of the officers from my regiment would buy the book and be very anti it. I have had four officers who have bought it and said that it's brilliant – 'Yes, we really are like that.' That really surprised me."