Chris Bellamy: The first rule of a just war... express your aims

Many westerners – and quite a few non-western Muslims - believe that, unlike the invasion of Iraq in 2003, the intervention in Afghanistan was, broadly speaking, a 'Just War'.

Afghanistan was a safe haven for al-Qa'ida, whom the Taliban were sheltering. Privately, many senior officers and other members of the great and good in the west believe that Iraq may be irremediable but that Afghanistan is, firstly, justified and, secondly, still, (just) winnable.

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But all is clearly not well in Afghanistan. When, since 2001, has the British Government clearly stated a strategic objective for our forces – or any forces – there. If you look for one, it will be hard to find. A strategic objective is one that can be stated in political or economic terms.

If you fight a war, you have to know what the war is for. Previous Afghan wars (Britain's wars in 1838–42, 1878-80) were about regime change. Putting a more favourable regime in charge, which meant one that did not favour the Russians in this crossroads of Asia.

'Regime change' was also clearly an objective in 2001. The aim was to bring democracy and prosperity, but you can't do that by taking the people's source of income away. And what was that source of income? Opium. Afghanistan was the world's greatest source of the white poppy, and western Governments were determined to be rid of it. But what do the people grow instead, apart from hatred and resentment?

Another was to bring humanitarian aid to a country at war for nearly 30 years, and to import some western ideals, including women's rights. Was that really realistic? Another aim was to instal a firm central Government that could control the country, in a country that has never respected or responded well to centralised Government.

A disparate land, with disparate tribes, and disparate warlords. Finally, the aim was to be rid of the Taliban – the movement that grew from a bunch of theology students, but which rose to provide, probably, albeit briefly and cruelly, the most effective central Government Afghanistan has ever known.

The new British Ambassador to Kabul has said that the UK needs to be in Afghanistan for perhaps another 30 years. Assuming the British Governments over that time have the endurance, the bottle and the continuity to maintain that commitment, he is probably right.

The military situation in Afghanistan, and particularly the south and east, is fraught. The initial intervention against the Taliban was largely dependent on the Northern Alliance, who were mainly favourable to the Russians in 1979-89. It was naturally assumed that the Taliban would put up a guerrilla or, as current jargon has it, 'asymmetric' opposition, but in recent years we have seen well dug-in and skilfully defended Taliban defensive positions taken on by Nato troops who were, ironically, inadequately equipped for what they should be good at – 'symmetric' – armoured and artillery attack – on fairly comparable adversaries.

The loss of an RAF Nimrod Maritime Reconnaissance Aircraft, operating out of Dohar, reinforces the impression that our 'symmetric' resources are getting stretched to deal with a supposedly 'asymmetric' adversary. Clearly, those terms are complementary. In the end, war is war. We need to clarify and publicly express our aim. If we do, it is likely to be a long one.

Chris Bellamy is Professor of Military Science and Doctrine at Cranfield University. His latest book, Absolute War: Soviet Russia in the Second World War is published by Macmillan on 20 July.