The great sixth century BC military strategist Sun Tzu wrote: “Strategy without tactics is the slowest route to victory. Tactics without strategy is the noise before defeat.”

With fighting in Afghanistan now entering its seventh year, no agreed international strategy, public support on both sides of the Atlantic crumbling, Nato in disarray and widening insecurity in Afghanistan, defeat is now a real possibility. The consequences for both Afghanistan and its allies would be appalling: global terrorism would have won back its old haven and created a new one over the border in a mortally weakened Pakistan; our domestic security threat would be gravely increased and a new instability would be added to the world’s most unstable region.

David Miliband, the British foreign secretary, is right – in the face of these consequences, withdrawal is not an option.

But then neither is continuing as we are. So what should we do?

Some say more troops should be sent and they are certainly needed. Some say those Nato members who are not sharing the burden of the fighting should do so – and they should. Some say we need more aid – and we do. We are putting into Afghanistan one 25th the troops and one 50th of the aid per head of population that we put into Kosovo and Bosnia.

Increasing resources in Afghanistan is clearly necessary, but it is not sufficient. Even if we were to provide what was necessary, and even if everyone pulled their weight, we would still find it very difficult to turn the tide, which is now running increasingly strongly against us.

What we lack above all is a strategy that all (including, crucially, the Afghan government and the international military) can buy into. We know well enough what the objective is – to help President Hamid Karzai’s government to govern so that we can hand over the tasks we are doing, including the fighting, to them.

However, we have not yet turned this aim into a plan. Neither have we agreed a single person to head up the fractured international effort, with the authority to bash international heads together and provide the support the government of Afghanistan needs to begin winning again.

Here is the plan I assembled over the past four months, as I reluctantly considered what I would do, if I had had to do this job.

Firstly, we (the international community) have to concentrate fiercely on the necessary and not be distracted by the merely desirable. To have too many priorities is to have none.

I fixed on three priorities for the period ahead.

The first is security. We have to convince ordinary Afghans that their government can provide them with better security than the Taliban. I do not mean here just military security – it is human security that matters. That includes electricity, the rule of law, effective governance and the chance of a job in a growing economy.

What is needed to deliver this is a much closer co-operation between the military and the civilian side. It is no good soldiers winning a battle with the Taliban if the civilian reconstruction takes too long to begin to improve the lives of the people afterwards. We British have a tendency to be rather self-congratulatory about our skill at this and a bit sniffy about our US allies’ hamfistedness and clumsy use of force. But it is very foolish to underestimate the US military’s ability to learn lessons fast, just as they did after Vietnam. US counter-insurgency practice is now as good as the best – and better than any when it comes to getting the civilians in straight after the military (the UK’s department for international development please note). We also have to start looking at security from a political angle. Breaking up the Taliban by winning over the moderates is a far better route to success than bombing and body counts.

Our second priority should be governance. Until we have strengthened the mechanisms of Afghan government we cannot ask them to do more: they cannot deliver what their citizens need and neither of us will be able to persuade Afghans that Kabul is a better bet for their future than the Taliban. We should make improving governance the first, and if we can the only, priority for all future aid programmes.

Here, however, we hit a dilemma. According to its constitution, Afghanistan is a centralised state. But on the ground it is a highly decentralised one. Which end of the pipeline of governance should we start with? The answer is start at the bottom and work with the grain of the Afghan tribal structure.

The third priority, linking these two, is strengthening the rule of law, from the judiciary, to the police, to the security structures, to the penal code. Corruption is always endemic in countries emerging from war and Afghanistan, where drugs super-charge the problem, is no exception. Unless and until the rule of law is established there can be no safe democracy, no trusted government, no successful economy and no security for ordinary citizens.

We have not lost in Afghanistan. Indeed the more I looked at it, the more I could see positive things to be built on. But we will lose if we do not start doing things differently. What we need is a strategy, not a disconnected collection of unco-ordinated tactics. What we should not need is a Chinese philosopher from 26 centuries ago to tell us that.

Lord Ashdown was leader of the Liberal Democrats and high representative in Bosnia, 2002-2006. He was asked by the United Nations secretary general to be the UN’s special envoy in Afghanistan but was rejected by Mr Karzai