Britain’s MPs this week uttered one long howl of anguish over Afghanistan. Their immediate targets were Joe Biden and Boris Johnson, politicians who just happened to be on the watch when Kabul’s pack of cards collapsed. But their real concern was that a collective 20-year experiment in “exporting western values” to Afghanistan had fallen into chaos. MPs wanted someone other than themselves to blame. A politician is never so angry as when proved wrong.

Like their fellow representatives in Congress, MPs somehow hoped the end would be nice and tidy, with speeches and flags, much like Britain’s exit from Hong Kong. Instead, tens of thousands of Afghans who had lived in an effective colony under years of Nato occupation had come to believe the west would either never leave or somehow protect them from Taliban retribution. They were swiftly disabused.

In 2006 I stood at dusk on a castle wall overlooking Kabul with a young UN official. He had just heard the Kandahar road was no longer safe. “Why,” he sighed, “can’t Afghanistan be more like Sweden?” I tried to see if he was smiling, but he was grimacing. For another 15 years, armies of western soldiers and civilians hurled stupefying amounts of money at the country. They created a wildly corrupt western dependency, where some 50,000 Afghans have links with the west that are now lethal. As for the “western-trained” army, one of its trainers told me it was mostly for show. An occupying power could not possibly motivate local youths to kill their fellow countrymen who might soon be ruling them. He rightly predicted: “They will just walk home.”

It is now 22 years since Tony Blair gave a speech in Chicago lecturing the US on his doctrine of international intervention. He wanted the west to invade countries across the world not in self-defence, but to save people everywhere from oppression. It was a reformulation of Alfred Milner’s Victorian concept of moral imperialism. British politicians on both the left and the right have long been uncomfortable about the abandonment of Milnerism as the acceptable face of empire. Global policing is somehow embedded in Britain’s political DNA. All Blair’s wars of aggression were cheered on in the House of Commons.

Many people have spoken this week of the “decline of the west”, lamenting the collapse of US moral authority. Yet these theories are beside the point. The belief that our moral values are somehow meaningless unless they are enforced upon those who do not share them is imperialist bigotry. It also leads to absurd biases. Iraq is now thought of as “bad interventionism”, as opposed to Afghanistan’s “good” version. The virtue of the latter invasion led President Obama in 2009 to bless the war in Afghanistan with a “surge” of soldiers, taking the US total to 110,000, mere target practice for the Taliban.

American gunboat diplomacy, initially supposed to salve the wounds of 9/11 in 2001, opened the door to fake morality and a trillion-dollar nation-building fantasy. The catastrophic return of Taliban autonomy became its inevitable conclusion. The US – with Britain as its lackey – committed liberal interventionism’s cardinal sin: half-heartedness. The craving to intervene is always followed by a craving to withdraw. Traditional empires at least pretended they would never leave. As it was, Afghanistan replicated departures from India, South Africa, Hong Kong and Iraq. If you invade and conquer an alien state, you own it, but must then disown it. Western rule has killed an estimated 240,000 in Afghanistan since 2001, more than the Taliban ever did. It has not left morality, just a mess.

We must assume strategists in Washington and London are now planning interventions in Taiwan and Ukraine against possible Chinese and Russian expansion. If you ask taxpayers to spend billions on defence, you need something to show for it. So you pretend, as Johnson did in his bizarre conversation with Biden this week, that “gains” were made in Afghanistan. You accuse non-interventionists, as did the former Tory leader William Hague, of demonstrating “the enfeeblement of the western mind”. In a recent column, Hague called on Britain to continue invading foreign countries when “our common humanity demands it”. In doing so, he sounded like Pope Urban summoning the First Crusade.

The concept of a global police force, so often cited, requires some framework of global consent. When the United Nations was founded, that consent was rooted in the first chapter of its charter. This stated that all members “refrain from the threat or use of force against the territorial integrity or political independence of another state”. On that basis it achieved general consent, if not always obedience. The end of the cold war and the assumed superiority of liberal western values emboldened the US and Britain to declare a “responsibility to protect” all those oppressed by their governments. The authority of the UN charter – rooted in national sovereignty – collapsed, and the UN gave way to the US as a self-declared policeman.

The American cold war historian Francis Fukuyama wrote recently that the US “is not likely to regain its earlier hegemonic status, nor should it aspire to”. There is no such thing as a global policeman. Individual nations best serve humanity by example or charity, not by war. Military intervention is rarely, if ever, humane. Western regimes have enough woes to confront in their own countries. If they crave moral outreach, it should be through the imperialism of ideas, of receptive minds and open doors, not of guns and bombs.

• Simon Jenkins is a Guardian columnist