It has been fashionable these last few years to view Afghanistan as the "good" war and Iraq as the "bad" war. This judgement is probably correct, but Afghanistan has been the right war fought in the wrong way, pursuing the wrong strategy, at the wrong time. What we now see as phase one of the Afghan war, the overthrow of the government in 2002, was brilliantly executed. From then, everything has gone wrong.

Firstly, we were distracted by the Iraq war, and largely left the new Afghan "democracy" under Hamid Karzai to its own devices. We turned a blind eye to the massive corruption – more than 20 families have become multi-millionaires from it in seven years. We ignored the monstrous growth of the Afghan poppy crop, with its legacy of heroin addiction and crime on western streets.

We did nothing to prevent the entrenchment of major and minor warlords running governmental baronies in pursuit of criminal gains, at the very real cost of ordinary Afghans. Finally, when we did intervene it was in a poorly planned, badly executed and under-resourced manner, calculated only to stir up resentment with little beneficial outcome.

We started with confused aims. We wanted to defeat the Taliban, although did not know which Taliban – the foreign radicals, the religious extremists, the criminals, the out-of-favour warlords or those with a real (and often justifiable) grudge against the government. Today, we are in principle willing to negotiate with all but the first of these.

In military terms, we massively underestimated what resources were needed to bring Helmand province under control, and we are still doing so. Inadequate numbers of soldiers have been exacerbated by deficiencies in equipment, from body armour to helicopters. We should not kid ourselves; equipment will not stop the inexorable count of lost and damaged lives. However, every life lost will be wasted if we do not have a clear view of what we want as a final outcome, and how to achieve it.

Our primary aim is to deny would-be terrorists a place in which they can organise and train, and from which they can strike out at our citizens. To do that, we have to have a strong and stable Afghan government, and to do that we need a people who are comfortable with their regime. That does not demand a Jeffersonian democracy, with all the appurtenances of the liberal state, from universal education and healthcare through to strong civic standards and gender equality, much as we would all like that. All it needs to deliver is justice and security to ordinary Afghans. If it can do that, the rest will come in due course.

This may sound more reminiscent of government in the Roman Empire rather than the 21st century. Regrettably, at the moment, we cannot even deliver that. In the Commons this week, Gordon Brown said we need to deliver justice on the ground. Quite right. He went on: "Not the medieval brutality of the Taliban, but the rule of law." This, I am afraid, shows he does not know what is going on.

Last October, I was in Afghanistan and I talked at length with people from Helmand province. What they told me was horrifying. Put to one side the incessant tales of theft, drug running, kidnap, extortion, rape and even murder by the Afghan National Police themselves. What they also told me was that the ordinary Afghan citizens seeking justice had two options. One was to bribe the national justice system to get a trial in perhaps a year, then eventually bribe the judge to get the outcome they want. Alternatively, they would walk five or ten miles to a Taliban village, get a hearing in a day and a fair ruling in a week.

As for security, our government does not appear to have any concept of what is needed to build a secure state. The current plans envisage about 134,000 Afghan soldiers and about 90,000 police. Iraq is a smaller country than Afghanistan, is less complex, either geographically or tribally, and has a history of stable central government. It has over 600,000 security forces, nearly three times those planned for Afghanistan.

So, it is time our political leaders got a grip. Our brave young soldiers will go on winning every battle, but the cost in lives will be tragically wasted if the political classes throw away the war. We need more coalition soldiers on the ground, of course. We need more equipment, no doubt. But most of all we need an achievable model of a stable, secure and decent Afghan state, and an idea of how to achieve it – and today we do not have that at all.

The author was the Shadow Home Secretary from 2003 to 2008