The crucial question on Afghanistan today is not whether this war is important. It is. It is not whether the consequences of failure are serious. They are. It is a much more brutal question: can we win? And the answer is no. Unless we change both our current policies and our present attitudes, failure is inevitable.

The reasons are manifold. The international community continues to lack a united strategy with clear priorities. Nato is all over the place. President Obama's plan is taking too long to be applied. British soldiers are fighting the war at full capacity, but their government is not. Respect for the Karzai government, to which we are tied, is not rising, but falling. We lack a political plan that works. And it is far from clear that the military plan is working. All of this has led to public support for the war eroding at a frightening rate.

There are no quick fixes. But we need to start immediately, to forge a co-ordinated response to each of these problems and, above all, to show the strategic resolve to see it through.

The central failure is the absence of any clear international strategy. The British think Afghanistan is Helmand; the Canadians think it's Kandahar; the Dutch think it's Uruzgan; the Germans think it's the north; and the Americans, until recently at least, thought that the only solution was a kinetic one.

Gordon Brown and his European allies have called for an international conference to review progress. This will be a waste of time if it does not produce the single united international strategy that has so far been so disastrously lacking. Nato, too, has to wake up to the fact that it faces a catastrophic failure with very wide consequences for its own future, unless it can start working like an integrated military alliance, rather than a hotchpotch of the committed and the half-hearted.

We now have an Afghan military team of the highest quality in US Generals Petraeus and McChrystal, recently joined by Britain's new head of the army, General David Richards, the first person to hold that post with actual – and much admired – command experience in Afghanistan. There is a chance for a new start. But the word from Washington is that Dick Holbrooke is floundering and the political plan is taking far too long to put together.

Some say the fault lies in Washington infighting, with Holbrooke imprisoned in the state department and ignored by the defence department and the CIA. Others say the problem is Holbrooke's personality. Whatever the reason, there is a perception of lack of co-ordination and drift from Washington.

President Obama's white paper on Afghanistan in March was excellent. But why is it taking so long to be properly implemented? Much rests on General McChrystal's imminent, long-anticipated military plan. He should propose a change in strategy and a change of gear.

The British government needs to change gear too. Brown's recent speech should have been a clarion call to the nation. Instead, it was a lecture on post-rationalisation. We all know our prime minister will never be Henry V at Agincourt; his chief means of persuasion is not charisma, but volcanic grumpiness. Nevertheless, he must find better means to tell us what this war is for, if he is to reverse the alarming erosion in public support. The British people are not squeamish. They have shown time and again that they are prepared to put up with pain and sacrifice, provided that they are convinced of the cause and see a reasonable chance of success.

You cannot win a war on half-horsepower. The prime minister needs to make it clear that this struggle is now the nation's first priority and we will strain every sinew to win it. In most of our recent wars, prime ministers have formed a special war cabinet. Why not now? Why not a minister for Afghanistan? Why have we not assembled the very brightest in the FCO, DfID, MoD and Cabinet Office to form a co-ordinated team to see this thing through?

This war will not be won by the bomb and the bayonet. It will be won by development and local ownership. So why is increasingly prosperous India at the top of Britain's aid list, receiving more than twice as much money as the ever more dangerous (and grindingly poor) Afghanistan?

We also need to think again about the Afghan government. If, despite the cloud hanging over the election, President Karzai is returned to power, we have to ensure that Karzai II is very different from Karzai I. His government must not be made up of the unfragrant coalition of war lords and crime bosses he put together to get himself elected. It should be a genuine government of national unity, which will clean out corruption and pursue an aggressive policy of integration of those Taliban who are willing to pursue their aims through the constitution, not the gun.

This should include a recognition by the international community that a programme to strengthen local government, running with the grain of Afghanistan's tribal structures, will be more effective than pouring more money into government in Kabul. Tribal politics are the key to Afghanistan, not western models of centralised government.

We must also take a long, hard look at our military tactics on the ground. The policy of "clear, hold and build" in rural areas might have worked three years ago, but since then, the situation has moved heavily against us. Now, in the rural areas at least, we are no longer fighting an external insurgency, but, for most of the contested rural areas of Helmand and Kandahar, a war among the people.

The aim of Operation Panther's Claw was to resurrect our lost opportunity. The theory was that, if our troops moved in, there would be a spontaneous reaction from the locals, abandoning the Taliban and seeking our protection and development. But in most cases it hasn't happened, leaving our soldiers once again overextended and isolated in Beau Geste-style forts, from which they can only dominate an area large enough to increase their vulnerability to ambush and roadside bombs, but too small to begin the development process.

If this is so, it's time to consider plan B. One option would be to concentrate our forces in the cities in future, so as to deepen the effect of the development process where it matters most, and then build out from there as force levels and resources allow.

Beyond that we may even have to consider plan C, a modern version of the old policy of Lord Curzon, but run from Kabul instead of Calcutta, which would use air power and special forces to prevent the Taliban ever again marching on Kabul or becoming a haven for al-Qaida, while we concentrate on the rest of the country outside the Pashtun belt.

All this will be very uncomfortable, but not as uncomfortable as reinforcing failure with more lost lives. It is not yet lost in Afghanistan. Not quite. We are in the territory of the last chance. There will be no more.