Decision to suspend joint Nato-Afghan operations clearly came as surprise to defence chiefs and strikes at heart of UK strategy

For months, if not years, Britain's defence chiefs have made it clear they are desperate to get out of Afghanistan. They have clung to the increasingly implausible claim that training and mentoring Afghan security forces was going well, indeed better than expected.

That claim has been shattered by Nato's decision, taken at the behest of the US, to suspend joint Nato-Afghan ground operations. The decision strikes at the heart of Nato and British strategy.

Worse, as far as Philip Hammond, the defence secretary, is concerned, officials have made it clear that the decision came as a complete surprise to him and his military commanders. Only on Monday Hammond, summoned to the Commons to answer an urgent question, told MPs that the latest wave of "insider" or "green on blue" attacks by Afghans could not derail progress.

He made no mention of the possibility of a decision to scale down joint operations with Afghan forces, even when such a possibility was raised specifically by the Conservative MP and former commander in Bosnia, Bob Stewart.

On Tuesday morning Hammond did his best to try and play down the significance of the decision by describing the Nato announcement as only a "draft order" that would have "minimal" impact on British operations in Afghanistan.

That claim sits rather awkwardly with repeated emphasis on the importance of the Afghan training programme, the very raison d'etre, according to the British government, of the continuing presence of British troops in the country.

It is understandable that Hammond would want to downplay the Nato decision, though it is likely to have a real impact on the morale of British troops, already affected by the recent attacks. Two soldiers from the 3rd Battalion of the Yorkshire Regiment were shot on Saturday as they went to help a man wearing the uniform of the Afghan police militia. He claimed to be injured before opening fire on the soldiers at a checkpoint in Nahr-e-Saraj in Helmand province.

Yet the real reason for the deployment of British forces in Afghanistan was to ensure that Britain's real enemy – al-Qaida – is defeated there.

But that threat to Britain's national security has been long gone. Hammond appeared to admit as much last week. "We have to be clear why we came here in the first place," he said in an interview with the Guardian. Now that al-Qaida had been "eliminated" from the country, it was not right to ask British troops to put their lives at risk for nation-building.

"We can ask troops who are here to help build a better Afghanistan, but we cannot ask them to expose themselves to risk for those tasks," Hammond continued. "We can only ask them to expose themselves to risk for Britain's national security, which is what they signed up to do."

The UK had "not come here to defeat the insurgency". In that case, it may reasonably be asked, why are any British troops still putting their lives at risk in Afghanistan?

There is growing frustration – to put it no more than this – on both sides of the Commons at the confusing messages coming from the very top of Britain's defence and military hierarchy.

Hammond insists British strategy has not changed. "We have got a strategic plan," he said on Tuesday. "We are working towards an end to our combat operations in 2014." It is as simple as that, come what may.