Four hundred and forty British troops are about to leave for Afghanistan to help fight the Taliban. Could someone explain why? It is now four years since British troops “left” that country, after 13 long years and £40bn of fighting, culminating in defeat in Helmand. In 2014, Nato formally ended its retaliation for Afghanistan harbouring Osama bin Laden. It said it would withdraw and leave a new Afghan government to its own devices.

Yet not one British politician – Conservative or Labour – can bring themselves to challenge Britain’s continued involvement in this war; including this month’s doubling of troop numbers. All are afflicted with that old post-imperial tic, wanting to punch above their weight, set the world to rights, not seem weak in public? Afghanistan is not a defence policy, it is a rash, a virus, a disease of misused power.

Donald Trump is suffering from it too. In one of his more enlightened campaign pledges, he promised to withdraw America from these pointless entanglements, in Iraq, Syria and Afghanistan. But when the defence lobby challenged his masculinity he caved in. As with Barack Obama, withdrawal turned to surge. Now Theresa May wants to hold hands. She feels she must send more troops too. She promises they will not fight. They may just die.

We are back to 2005, hoping that the Kabul regime, drenched in a staggering trillion dollars of American money, can still gain control of the country, of which some 70% is controlled by or vulnerable to the Taliban, drenched in opium revenue. Now, as then, Nato spin-doctors talk of training the Afghan army. They talk of a political solution. They talk of talks about peace just over the horizon. But they cannot control a single province safely, least of all the opium belt of Helmand.

The assigned task of new British troops is to ferry international workers around a supposedly safe Kabul in huge people carriers, dubbed “armoured Uber”. They are not soldiers but police support officers. As for training the Afghan army – as anyone who has witnessed it can attest – it has been about to “turn the corner” for a decade. The army is demoralised, under-strength, and cursed by desertion. It is cannon fodder for a Taliban that has been fighting Kabul for generations. The idea that British lives must be risked aiding such a force is absurd.

What this has to do with the defence of the realm is a mystery. This week’s 440 troops were nothing more than a gift from May to Trump during his diplomatically inept visit to Nato last month. British defence policy has become a goodie-bag. And the Ministry of Defence has the cheek to claim it is short of money.

• Simon Jenkins is a Guardian columnist