By Frank Gardner
BBC News security correspondent
The planned reduction of 800 British troops serving in Iraq announced by the UK Defence Secretary John Reid is not the first such withdrawal nor the biggest.
British troops have come under increasing insurgent attack
The Ministry of Defence says there have now been five such reductions and this is the largest since 2004. But it is significant.
The number of troops leaving Iraq amounts to a reinforced battalion, or 10% of UK forces deployed there on what the Army calls Operation Telic.
Officially, the reason given for this reduction is progress in Security Sector Reform (SSR), the process of training up Iraqi military units to maintain law and order and tackle any insurgents.
In recent months this process has been proceeding rapidly, with Mr Reid telling the House of Commons that Iraqi government forces numbering 230,000 are now trained and equipped, with a further 5,000 being added every month.
However, even in the Shia-dominated south-east of Iraq, where British forces are concentrated, the security situation is not as placid as British commanders would have liked.
British patrols have frequently been ambushed by deadly roadside bombs and there are signs that Iraqi police forces have been heavily infiltrated by militia factions.
The withdrawal amounts to 10% of British troop strength
But there are compelling strategic reasons why Britain wants to start "drawing down" (ie reducing) its military presence in Iraq.
The relatively small British Army is now heavily stretched by its commitments in the Middle East, the Balkans and now Afghanistan, where up to 5,700 troops will be deployed for the next three years.
There is also the question of Britain's global reputation, especially in the Arab and Muslim world where the US-led occupation of Iraq is deeply unpopular.
Despite the fact that Iraq's political leaders have yet to ask Western forces to leave, there is a widespread perception in the Middle East that Britain and the US want to remain in Iraq as "neo-colonial occupiers" intent on taking its oil.
Reducing British troop numbers, and in due course handing over security of the southern provinces to Iraqi forces, will go some way towards redressing that perception.