When we will we ever learn? How many more young British soldiers do we have to send to be slaughtered, maimed or traumatised in futile foreign calamities at the behest of US presidents? In the buildup to Iraq and Libya, critics were ridiculed as naive peaceniks or demonised as the heartless useless idiots of former western clients Saddam Hussein and Muammar Gaddafi.

What came next? Hundreds of thousands perished, millions were injured and displaced, while extremist jihadists flourished. With the British government preparing to bend to Donald Trump’s demands to double troops in Afghanistan, there are no excuses. We know the Afghan conflict – the longest in US history – has been a disaster. This is a blood sacrifice for the megalomaniac in the White House, paid by young Brits, teenagers among them.

Before Trump assumed the presidency, it is notable that he described the war as a terrible mistake

According to a poll, 56% of Britons believe that military involvement in Afghanistan “has not been worthwhile”; just 25% take the alternate view. Public opinion does not always mean wisdom, so let’s judge if they’re correct. 456 Brits have perished in the conflict; and – according to one study – more than a quarter of the 220,000 personnel who served in both Afghanistan and Iraq have been left wounded, ill or psychologically harmed. The US has expended a trillion dollars on this calamity; it may have cost Britain tens of billions.

What do we have to show for it? More than a hundred thousand dead Afghans, tens of thousands of civilians among them, and yet the government control just 56% of the country’s territory. The Taliban, it is estimated, are active in 70% of the country. Opium production reached a record high last year. According to the former western-backed Afghan president Hamid Karzai, the war is a failure. “We have more radicalism, we have more extremism, we have more attacks all around,” he says, accusing the west of “targeting Afghan homes, Afghan people and not the sanctuaries outside of Afghanistan”.

Before Trump assumed the presidency, it is notable that he described the war as “a terrible mistake”, a “total disaster”, and a “complete waste”. Now he is not only sending his own young citizens to die and suffer for this “terrible mistake”, but demanding we send our own. Where are all those supposedly moderate, sensible commentators who decry Trump as a demagogic menace to democracy and peace? Will they speak out, or is Trump only a quasi-fascist danger until he wants to engage in war, and then he becomes presidential, sensible, a statesman even?

The victims of Trump are barely remarked upon. What of the western-backed Saudi slaughter in Yemen, and the sixfold increase in US airstrikes there under Trump? What of the 215% increase in civilian deaths because of US-led airstrikes in Iraq and Syria under Trump? What of Trump fuelling violence in occupied Palestine? This man should not be allowed to dictate British defence policy. We cannot allow our government to casually toss away the lives of our young simply because Trump demands it.

• Owen Jones is a Guardian columnist