The images plastered across our screens in recent days of Afghan civilians at Hamid Karzai airport desperately trying to flee the country, as well as the bombing on Thursday, have been heartbreaking. And, for me, slightly surreal. I lived in that airport while serving as a soldier in the British army. Watching the scenes of chaos on the tarmac, my first thought was for the civilians who worked inside the airport. I spent many afternoons after work sitting in the Afghan-owned Istanbul Cafe, a ramshackle building of several floors that overlooked the airfield, drinking the strong coffee prepared by woman and men I came to know and respect. Their journey into work every morning was 10 times as dangerous as anything I ever did in Kabul. They ran the gauntlet on bicycles while we took shelter inside armoured vehicles. I cannot help but wonder: are they safe? Did they get out?As the shock of what had happened subsided, and the Taliban raised their flag above Kabul, I grew resentful and angry, thinking about why the mission in Afghanistan failed and whether it could have gone differently. I worked as a soldier at the coalface of Nato’s Resolute Support mission, which was supposed to train, support and assist Afghan security services and institutions. We provided security for advisers while they engaged with their Afghan counterparts in Kabul. Generally, this would mean picking them up, taking them to the meeting, providing security for the meeting and bringing them back to base. From my perspective, there were at least two fundamental errors in the mission’s approach. The first was the massive outsourcing to the private sector that underwrote the operation.Let me state categorically, our service people are not to blame. The soldiers I served with, those who fought in Helmand and later safeguarded in Kabul, acted, almost without exception, with the utmost professionalism and valour. To have served alongside them is the great honour of my life. But Nato’s mission was not fit for purpose. When I was in Afghanistan, private military contractors numbered almost 30,000. Some were engaged in protection tasks, but many more were responsible for training and mentoring Afghans who held positions of significant influence. They advised on intelligence, war-fighting, diplomacy, policing, you name it. Some of them were doing their best. Many more didn’t give a damn. Many were on six figures and had been for years. Afghanistan for them was a cash cow, a way of putting their kids through college (most were American) or paying off a mortgage. In sum, there were too many poorly qualified people working without accountability, getting paid far too much. If you want an answer to the question of why Afghanistan’s military crumbled in weeks, take a long hard look at their so-called mentors. Then there was the simplistic assumption that everyone in Afghanistan could fall into two categories, enlightened liberal reformers who would welcome a western presence, and conservative folk susceptible to the Taliban. Needless to say, things were more complicated than that.There were some pretty unsavoury characters who worked with us in Kabul. One morning, an interpreter who had worked with the British for decades sidled up to me at breakfast and pointed at a young Afghan woman who also worked as an interpreter. In a voice loud enough for her to hear everything, he declared her a “filthy whore”. His reason? She was wearing a pair of jeans and a bright pink headscarf. This sort of language and these attitudes were commonplace and generally went unchallenged by soldiers and contractors, who didn’t want to be seen as undermining locals. And if they were accepted in a Nato base, what hope was there of combating the Taliban’s brutal misogyny? Corruption existed at every level. One afternoon I provided protection for a meeting between an Afghan air force lawyer and his US adviser. As I sweated into my body armour, they discussed an investigation relating to unauthorised travel on Afghan air force flights. In brief, the Taliban had been able to board flights reserved for Afghan soldiers and fly across the country with impunity. After several hours of back-and-forth on how best to proceed, the American eventually lost his cool and shouted: “You have to get rid of these [corrupt] people!” The Afghan lawyer calmly answered: “Would you like me to disband the entire Afghan air force?” The American had no answer to that. The west has had no answer to that for 20 years. How Nato believed that these fragile institutions were capable of holding back a group like the Taliban, who spoke with one voice and strove towards one end, is beyond me. In truth, it probably didn’t – it was accepting of Afghanistan’s fate and the fate of its hopeful youth. In what we call the combat estimate, we ask ourselves a number of questions. One of them is, “What resources do I need to accomplish each effect?” Essentially, troops to task. Nato got this one wrong. It did not need 30,000 self-interested mercenaries who cared more about their bank accounts than the future of Afghanistan. It needed a small and dedicated grouping of experts supported by an appropriately small and well-equipped protection force. This, coupled with virtual engagement (if Covid-19 has taught us something it’s that we can work at distance), would have had the same impact as the bloated mess that Resolute Support eventually became. Importantly, maintaining boots on the ground, albeit in a limited capacity, would have sent a clear message to the Taliban: these new Afghan institutions do not stand alone. To those who say that our presence was unwanted and fruitless, to those who claim that we could never hope to help change Afghanistan for the better, I would ask them to take a look at the videos of the young desperately clinging to the undercarriages of C-17s. Take a look at the figures establishing themselves in offices once held by the democratically elected. Take a look at the devastating attack by the Islamic State. Our presence was enough to stop all of this. Sometimes preventing a change is as important as instigating one. To those whom we promised a future, we must now open our arms. Lives are at stake. Cost is irrelevant. We must do what we can. The author was a soldier in Nato’s mission in Afghanistan The fall of Afghanistan: join a Guardian Live online event with our journalists Emma Graham-Harrison, Peter Beaumont and Julian Borger analysing the latest developments. Monday 6 September at 7pm BST. Book your tickets here. All profits will be donated to relevant charities.