Three years ago, in Helmand, I watched Nick Clegg present a battle plan to the British military. Unfortunately, it seems to be following it. The plan was a crayon and felt tip scrawl by one of his sons, who'd made his father promise to give it to the army. Handed over to amuse, it suggested that the baddies hidden beneath mountains could be fought by a few soldiers piling from a helicopter. We smiled at the juvenile simplicity.

Now, in Helmand, the military are doing just this. They call their murderous night raids against insurgents a bold strategy for success, when really the intensification of violence is evidence of failure. We are, as David Miliband will warn in a speech on Wednesday, trapped in a war with no plan other than to kill as many baddies as we can before fleeing.

At the end of my trip to Afghanistan with Clegg and Nick Harvey, now the armed forces minister, I wrote an overly optimistic piece suggesting that the army might be about to turn things around. Smart soldiers using jargon deployed PowerPoint charts to prove it. It seemed wrong not to take their confidence seriously, and allow them time to make their plans work. I did. More importantly, ministers did.

They have had the time and the plans didn't work. Almost everybody in politics thinks privately that military involvement in Afghanistan has been a disaster. The pity is few dare say so.

Afghanistan is already yesterday's war, though it is still to be tomorrow's defeat. Mentally we have adjusted for the end, though there are still 9,500 British troops in action. Many soldiers and marines are on their fourth tour of duty – two years of a young adult life. Some face redundancy on return. We've been in Afghanistan for 10 years, and in Helmand for five – world wars were fought and won in less. It's becoming one of those conflicts which seem to have no beginning and no end and probably no point, slipping from our enthusiasm and into history. Libya is eating up our energies instead.

There was little interest last month when the foreign affairs committee published what (by its standards) was a strong criticism of the military surge. Perhaps some attention will be paid to Miliband's speech. Of all politicians involved in pursuing the war he has been the bravest in speaking out. His intervention, as with his previous ones, is being made in America. That's where decisions are being taken. Britain, having set 2015 as a date for withdrawal for no reason other than the proximity of an election, is ticking off the days on the walls of Camp Bastion like a prisoner scratching out a sentence.

When General Petraeus leaves Afghanistan later this year he will of course claim to have broken the back of the insurgency, but what he has really done is scatter it across the country in response to ultra-violence. His predecessor, General McChrystal, promised to pacify 40 districts by last December and 40 more by the end of this year. It hasn't happened. Talk of stabilising Kandahar has come to nothing – those mega-operations which were supposed to drive the Taliban out of their capital. In the north, Mazar-i-Sharif has rioted against the UN. In the south, we are indulging the fantasy that Lashkar Gah can move to Afghan control. Across the country, the coalition is more feared, violence higher and the president, Hamid Karzai, more unpopular than ever.

Read Rodric Braithwaite's magnificent new book Afgantsy to see where this will lead. His compassionate and brilliantly researched account of the Soviet experience in Afghanistan tells the story of an almost accidental invasion which collapsed not because of any single defeat but because the occupation became too expensive and incoherent to sustain. We are following the same path, and though Braithwaite is too discreet to make the comparison, the Soviet occupation was arguably more successful than ours. It ended in a negotiated settlement which might have lasted if the west hadn't funded its ruin.

The obvious thing to say – and when it's obvious you have to ask if there is a problem with it – is that we must talk to the Taliban. Without that, we will leave a broken country. Our present strategy, says one official who has been at the heart of it, "is all a big, big lie". Miliband will urge talks this week and of course he is right. But here's the problem: what if no one answers? The Taliban have little incentive to reach a deal. A few hopeful signs – a half-recanting speech by the previously obstructive Hillary Clinton – does not yet amount to a process.

In the meantime we are turning Afghanistan into a hyper-militarised state, funding a vast army and a security service which is becoming a government of its own. In the sham defence of democracy we will leave behind authoritarianism. It will no doubt last for a while after we go, as President Najibullah did after the Soviets. The Taliban will find they are not strong enough to rule Afghanistan. But nor is anyone else. If our present war is a calamity, what follows it will be worse. This is no way to end a column and no way to end a war, but maybe we will just have to shrug our shoulders and go.

• Jon Snow chairs a Guardian and British Museum debate on Afghanistan at the British Museum tomorrow. Tickets at www.britishmuseum.org