Britain will be sending up to 4,000 troops to Afghanistan in a move which will in effect mean fighting wars on two fronts with forces still deployed in Iraq.
The Government had planned for sizeable withdrawals from Iraq by the time the Afghan force is sent in April and May. But with no sign of peace breaking out in Iraq, military commanders have now ruled out that possibility.
The size of the force for Afghanistan, just 1,000 fewer than at the height of the war five years ago, also shows just how much the security situation there has unravelled while the US and Britain invaded Iraq.
Senior officers say carrying out two such major operations is just about possible but only for a short time.
Yesterday it was confirmed that Prince Harry could face frontline service in Iraq or Afghanistan when he joins the Household Cavalry. The 21-year-old has chosen the Blues and Royals, which form part of the Army's oldest and most senior regiment, as the next stage in his military career.
But rather than focusing on ceremonial duties in the mounted division, the prince will train with the armoured reconnaissance unit, meaning he could see action in conflict zones.
The Blues and Royals, who were deployed at the start of the Iraqi conflict, are expected to be sent to Afghanistan in the future and possibly again to Iraq.
The Afghan deployment is potentially more hazardous for British forces than the current situation in southern Iraq, which has remained relatively quiet compared to Sunni central Iraq where the bulk of the US forces are based.
British troops will head for the province of Helmand in the south-east, which has become a bloody battleground with a resurgent Taliban and al-Qa'ida infiltrating across the Pakistani border.
The British troops will replace soldiers from America, which has recently lost more than 100 men - a larger number than the entireBritish forces killed in the Iraq war - in repeated attacks.
Worryingly for Nato forces, this has included a dramatic rise in suicide bombings - a tactic hitherto relatively unknown in Afghanistan but a favoured tool of the Sunni insurgents in Iraq.
The new Afghan force will be spearheaded by paratroopers from 16 Air Assault Brigade. The land contingent will be backed by Apache, Chinook and Merlin helicopters as well as a squadron of RAF jets based at Kandahar.
John Reid, the Secretary of State for Defence, is due to make an announcement about the Afghan deployment today following a meeting of the Cabinet.
The announcement comes in advance of next week's London conference on Afghanistan - co-hosted by Tony Blair, the UN secretary general, Kofi Annan, and the Afghan president Hamid Karzai - to formulate a blueprint for the country's immediate future.
However, the full extent of the British troops being sent may be temporarily withheld with anxieties that the Dutch Parliament is still to ratify the deployment of 1,400 troops due to join the British force.
The Defence Secretary is likely to stress that the British effort would be geared towards counter-narcotic operations and the establishment of a provincial reconstruction team in Helmand.
Mr Reid said in the Commons yesterday: "Although the principle of deployment to the south has already been announced, I will not announce the detailed deployment to the south until I am absolutely satisfied about three things.
"The first is that the British military configuration is sufficient to meet the task in hand, in the opinion of my commanders who advise me. The second is that the economic development aid and money is sufficient to offer alternative livelihoods and development if we are to tackle narcotics.
He added: "The third is that we have a Nato configuration of military troops around us which satisfies me ... I am satisfied of the first, I am satisfied of the second, but I am not satisfied of the third".
Liberal members of the Dutch coalition government are opposing the deployment, which is thought to be deeply unpopular among the public.Reuse content