By Jonathan Marcus BBC defence correspondent

The bulk of the 1,700-strong Royal Marine battle-group, which has been in Afghanistan since 18 March, is to be withdrawn by the end of July.

There will also be a scaling-down of Britain's contribution to the international security assistance force, or ISAF, in Kabul - from 1,300 troops to about 400.

Defence Secretary Geoff Hoon stressed the withdrawal does not imply an erosion of Britain's commitment to Afghanistan or to global war against terrorism.

But when any military operation ends it is appropriate to weigh up what it has achieved.

Mr Hoon makes 'no apologies' on mission

The two rather different British operations - one offensive, the other essentially peace-keeping - have had rather different outcomes.

One of the distinguishing features of the British military is it is one of the few armies capable of providing what might be called a "get-you-in" package.

From a standing start, Britain became the framework force around which ISAF was built.

Though its mandate was restricted only to Kabul, the successful holding of the loya jirga - the Afghan assembly which has just decided the next interim government - depended fundamentally upon the relatively secure environment created in and around the Afghan capital.

The offensive mission of 45 Commando proved more controversial, partly because of expectations it might become involved in some serious fighting.

This was due to initial briefings, flamboyant language from the force commander and, partly, a fair degree of media hype.

The newspapers, it seems, were spoiling for a fight with al-Qaeda as much as the Royal Marine commandos themselves.

But the deployment proved relatively uneventful.

'Low-key mission'

Their four operations destroyed cave positions and ammunition.

The Ministry of Defence claim they denied al-Qaeda and the Taleban the ability to operate in certain areas.

It also claimed it had some impact upon "winning hearts and minds" by distributing supplies and blankets to isolated communities.

There are lingering doubts in many experts' minds as to quite why the Royal Marines were sent to Afghanistan in the first place

But it proved a more low-key mission than expected.

Mr Hoon made no apologies.

"Those who carp about lack of action," he said, "do so from a position of ignorance about the nature of warfare."

That may be true.

But there are lingering doubts in many experts' minds as to quite why the Royal Marines were sent to Afghanistan in the first place.

Their mission seemed ill-defined at best.

It is instructive that defence sources now insist future operations must be based upon the availability of necessary intelligence and an acceptable concept of operations.

This hints, perhaps, that Operation Jacana was not quite all it was cracked up to be.