The sweet salvation of the summer recess over, we returned to Sandhurst for our final term of officer training. It was 11 September, 2001 – a day that started with a hike in the sunshine and which came to define my time in the British Army. The events of 9/11 would lead to my own deployment in Afghanistan and Iraq, and the loss of dear friends and comrades.

Of course, on that September morning none of us knew how events 3,000 miles away – and the political decisions taken in the aftermath of those terrible attacks – would have such a momentous impact on our lives. After the buses dropped us off, the hike started jollily. The 90 or so young men on the cusp of their twenties, at the peak of their fitness and powers, who in three months’ time would commission as officers into the British Army to lead its soldiers, were more like a gaggle of schoolboys. We just had to walk, and it wasn’t raining! We couldn’t believe our luck.

But as the day progressed, text messages on Nokia phones told of an incident in New York involving the World Trade Centre and maybe even a passenger jet. At the hike’s end, the cadets and directing staff crammed into a small pub. Watching the images on the television news, how could we realise that the scenes unfolding before us would shape and manipulate our years in the army yet to come?

It was some time before my own involvement in the ‘war on terror’. In the months after 9/11, I was deployed to Kosovo after all the hard and dangerous work was done and the conflict’s darkest days, when pregnant women were crucified with stomachs slit open, were long gone. The restored peace seemed to indicate military invention – the type that inspired me to sign up to the army in the first place – could work. Tony Blair certainly came to believe as much.

In 2004, I was sent to Al Amarah in south-eastern Iraq. It was the tour of 1st Battalion the Princess of Wales’s Royal Regiment, made famous (and infamous) by the Battle of Danny Boy, resulting in the Al-Sweady Inquiry; the siege of CIMIC House, and a Victoria Cross for Johnson Beharry.

I absolutely loved it there and suspect I wasn’t alone. We all tasted ‘the kingliness of Friendship’ that C.S. Lewis described reflecting on his military service. ‘We meet like sovereign princes of independent states, abroad, on neutral ground, freed from our contexts.’ We felt invincible: we had the services of Basher-75, the American C-130 gunship raining down fire from the night sky. It’s nice to feel you have Zeus on your side. That feeling wasn’t to last: two years after my first deployment, I was sent back to Iraq, this time to Basra, to deal with improvised explosive devices coming from Iran. My previous enthusiasm was fading; the security situation was deteriorating dramatically and the British military weren’t helping; some of my best army friends had called it and left; things were obviously going awry.

Finally, in 2009 Afghanistan called, and I deployed with the Welsh Guards Battle Group to Helmand. Our commanding officer, Rupert Thorneloe, was a lovely man, but he would not return home: he was sliced in two by an IED. That tour broke me – and I was ensconced at headquarters. Even from that relative safety, it was clear we were in a dire situation. The talk of loyalty, integrity, discipline and courage drilled into us at Sandhurst felt like a distant memory.

‘Hell is total separation from God, and the devil is the will to that separation,’ Aldous Huxley wrote. In Afghanistan we tasted that separation.

Twenty years since that hike, now there are the mounting suicides among veterans, the suffocating rage, guilt and shame; words and events that still catch in the throat; therapy sessions for PTSD and moral injuries; ruined relationships and fractured families, life chances forever lost in the fallout of 9/11 and the path to Afghanistan.

Mercifully, many soldiers have found that these painful scars can be healed with love and support; there is hope. Of course, a sense of humour helps too. So on the 20th anniversary of 9/11, I’ll flick a middle finger and a smile to our generals and politicians; with the other hand, I’ll offer Raymond Chandler’s gunman’s salute of outstretched thumb and forefinger accompanied by a respectful nod to Terry Taliban. You licked us, but no hard feelings.