For months after the allied invasion of Afghanistan in 2001, there were no Taliban attacks in Kabul. Now, as the weekend's gun, rocket and suicide attacks demonstrate, they are frequent and fatally effective. This is one measure of the progress of the war, more than 10 years on. There are many others.

According to a devastating account from a senior US army officer, the Taliban now range freely across much of the country. US forces barely control the territory they can see from their highly fortified bases. Lieutenant Colonel Daniel Davis reports that the Afghan army, like its government, is neither competent nor trusted.

The war was supposed to end with the Taliban arriving as supplicants to the negotiating table once sufficiently "degraded" by allied attacks. That strategy has been turned on its head. If anyone is a supplicant it will be the allies, desperate to make a deal they can claim as some kind of limited "victory" before they pull out. But, if this weekend's events are any indicator, the Taliban don't seem very interested in talking.

Where did it go so wrong? With the lucidity of hindsight (for I was once an enthusiastic supporter of the war), it appears that a fatal mistake was made before the campaign even started. In Bonn, an international conference was convened to construct a political strategy to follow the inevitable military victory. A moderate Pashtun would lead a government comprising all factions and ethnicities – or at least those we approved of. This new dispensation was to be endorsed by the traditional loya jirga, or "democracy, Afghan-style", as some chose to call it. It sounded plausible and everyone in the "international community" signed on.

But the new Afghanistan was in fact a fantasy.

A few weeks after Bonn, allied forces quickly drove out the Taliban – at least in parts of Afghanistan. The Karzai government was installed. The illusion persisted. But even in 2002, the flaws in the fantasy were evident for those who cared to look. Indeed they had been evident in Bonn.

For the one faction not invited to join the new Afghanistan was of course the Taliban. It was simply assumed that they would disappear. When I briefly served in the British embassy in Kabul shortly after the invasion, nobody ever asked why there was no allied presence in large swaths of the south. At the main airbase at Bagram, military operations were presented as mere "mopping up" of rag-tag forces driven into the mountains. But I remember one SAS commander who seemed less than convinced of this rose-tinted narrative. Perhaps he already saw the auguries.

Accompanying a senior official visiting our new allies in the north and west of the country (again, none in the south), everyone told us that the loya jirga would produce the wished-for democratic stability. Nobody mentioned that our partners in this project often behaved more like tyrants than democrats. One was reported to be given to tying the limbs of his opponents to two tanks then driving them in opposite directions.

The allies had all the guns and the money. Was it any wonder that any Afghan we met told us what we wanted to hear? At every meeting we were accompanied by squads of heavily-armed soldiers. As for the money, iIt was muttered only sotto voce at the Kabul embassy that cash-filled briefcases were regularly handed to new government ministers and warlords on "our side". Even nice Mr So-and-So, who spoke such good English and presented so well on TV, was on the take. Today, the only surprise is that we seem so shocked at the corruption of the Karzai government, given that we helped corrupt it.

The truth was that the allies were not creating a new democratic Afghanistan. Wwe had instead joined one side in a civil war that had raged for decades, has not ceased despite the allied presence, and will resume with full force once the western forces depart. It seems astonishing now that we were so wilfully naive. It all made such good sense at the time.

We entered Afghanistan and tried to make it comply with our fantasy, ignorant of its already complex realities. We occupied only small pieces of the country but declared that we had vanquished all of it. We constructed a new "democratic" order – but excluded those most likely to oppose it while including the brutish and corrupt (and then we corrupted them some more).

That these contradictions now seem so clear serves as a reminder of how stupid we were.