Petraeus briefing a matter of strategy

There are currently 5,500 UK troops serving in Iraq

General David Petraeus, the top US commander in Iraq, is due in London to brief UK Prime Minister Gordon Brown, fresh from his week giving testimony to Congress about the US troop "surge".

BBC diplomatic correspondent Jonathan Marcus examines Britain's role in Iraq.

After a week of briefings, hearings and photo-opportunities in the United States by both General David Petraeus and US Ambassador to Iraq Ryan Crocker, one thing seems clear - a draw-down of US forces in Iraq will begin soon.

But it is likely to bring US troop levels down only to pre-surge figures by the summer of next year.

US Defence Secretary Robert Gates has estimated that, in all probability, some 100,000 US troops will still be in Iraq when the next president enters the White House in January 2009.

Vital support role

So what does all this mean for Washington's closest ally in the Iraq enterprise, the UK?

It has just completed a major re-deployment of its forces in southern Iraq.

Gen Petraeus will set out the rationale behind current US thinking

On the 3 September British troops pulled out of Basra City, re-locating to a base at the airport.

There are some 5,500 British troops in Iraq who still have an important mission.

They are responsible for security in much of Basra Province, though such duties will progressively be handed over to local Iraqi forces.

Nonetheless British troops will still provide a vital support or over-watch role.

British troops also have important training duties and they patrol the border with Iran in Maysan province.

And there is another important strategic responsibility. The British sit astride the key logistical supply routes back into Kuwait.

Prime Minister Brown has stressed that Britain was staying in Iraq "to discharge its obligations to the Iraqi people and the international community".

All well and good, but what about any obligations to its US ally?

American spokesmen have made it clear that they expect the British to stay and finish the job.

This has caused some tensions with one or two retired US generals being less than tactful about the British performance.

Gen David Petraeus is likely to be diplomacy personified during his London trip, and offer praise for the British armed forces' efforts in Iraq.

'Strategic defeat'

But what the British government has sought to present as a rational and progressive handover of responsibility to the Iraqis is seen by many commentators in the UK as little short of a strategic defeat for the British.

They never had sufficient troops on the ground and the post-war climate was very different from that which had been expected.

Britain's role in Iraq is politically unpopular at home and Britain's armed forces are severely over-stretched, having faced intense combat in Afghanistan.

Many believe that current troop numbers in Iraq are unsustainable and that reductions to about 3,000 could come by the spring of next year.

In his private discussions with British Defence Secretary Des Browne and with the prime minister, Gen Petraeus will be setting out the rationale behind current US thinking.

But it would be surprising if he did not stress the US view of the need for a continuing UK troop presence on the ground.

This is not just a matter of appearances, though being deserted by its leading ally would have severe political repercussions in Washington.

It is also a matter of practical strategy as well. If the British go, then the US will have to find units from its own over-stretched military to bolster security in the south.

Those supply routes to Kuwait are vital for the Americans too and they also provide the best exit route for any eventual US withdrawal.