Controversial plans under which British troops would move from Helmand province, their main centre of operation, to the Taliban heartland of Kandahar and neighbouring Uruzgan have caused divisions within the UK's military and diplomatic hierarchy.
The planned redeployment is a key plank of the strategy of General Stanley McChrystal, the American commander of Nato forces in Afghanistan, to achieve a military victory before talks get under way with elements of the Taliban.
The Independent has learned that although the two most senior British commanders in Afghanistan are backing the proposed transfer, the head of the military, Air Chief Marshal Sir Jock Stirrup, believes it will be a mistake. So keen are the Americans for the British force to make the switch that Washington has offered to underwrite a sizeable part of the substantial costs involved.
General Sir David Richards, the highly influential head of the Army, is said to be "keeping an open mind" on the matter. He is keen to ensure that the Kandahar mission is not summarily ruled out and would like a feasibility study to be undertaken so that various options can be presented to whoever is in 10 Downing Street after the 6 May election.
The proposal was discussed privately among officials who attended a Nato foreign ministers meeting in the Estonian capital, Tallinn, at the end of last week. However, the political sensitivities involved, namely the impending UK election, has meant that no official comment has been made.
In another prong of Gen McChrystal's strategy, defence sources have revealed that the general is considering sending US forces to the Kunduz region in the north to counter a growing insurgency, despite the presence of 4,300 German troops.
The Germans have been accused of allowing the region, which had been relatively peaceful, to turn into another militant front by refusing to take adequate military measures.
One of the Germans' few aggressive moves turned out to be disastrous – air strikes were called in on two tankers hijacked by the Taliban, which resulted in the deaths of around 70 civilians and widespread Afghan anger. The air attacks were directly contrary to rules set out by General McChrystal in an explicit attempt to prevent civilian casualties. An attempted German cover-up led to the subsequent resignations of senior figures in the German military.
The plan to move the 9,500-strong British contingency has been necessitated by the refusal of the Canadian government to extend the mandate of its 3,000 troops in Afghanistan when it runs out next year. The Dutch force in Uruzgan is also expected to leave, creating yet another "hole" that Nato has to fill at a critical juncture in the war. The Americans already have around 20,000 troops in Helmand – almost twice the size of the UK contingent – and more are arriving. The US forces have already taken over swathes of the province, including previously British-held towns like Musa Qala.
Lieutenant-General Nick Parker, the British deputy Nato commander based in Kabul, and Major-General Nick Carter, who is leading the allied forces in the south from Kandahar, are among those who hold that the UK must answer the request from Gen McChrystal for a "real fighting force" in what is seen as the crucible of the Taliban jihad.
In recent weeks, the call has been forcefully backed by US General David Petraeus, who is overseeing both the Afghan and Iraq campaigns. Following the retaking of Marjah, preparations are under way for a similar mission in Kandahar this summer.
Although there will be targeted operations in Kandahar City, the main thrust of military action would be into outlying areas which have slipped back to insurgent control. Any UK force sent to the area would have the job of holding that cleared ground.
Proponents of the move say that taking on the Kandahar challenge will help heal the fractures in US-UK relations that date back to 2007, when British forces began drawing down from Basra, ultimately refusing American requests to stay on.
Opponents of the redeployment say it will mean abandoning the "blood and treasure" – military parlance for combat and financial investment – made by the UK in Helmand. Out of 281 British fatalities in the Afghan war to date, 250 have been in Helmand.
Sceptics of the switch, especially in the Foreign Office, also stress that valuable experience and knowledge of tribal and political complexities in Helmand would be lost at a time when some progress has been made.
Kandahar, they also warn, is unlikely to be a short-term commitment, which could be problematic at a time when the British public are increasingly questioning how long the deployment will last.
A diplomatic source said: "The military always felt unhappy about the justifications given for the Iraq war. They don't feel the same way about Afghanistan and some of them are quite Messianic. This is a lot to do with that. But many of us feel that we should stick to what we have in Helmand and not be over-ambitious."
However, one senior serving general countered: "We were hardly going to stay in Helmand forever, these are not the days of the Raj. There is no point in being 'Little Englanderish' about it. This is a Nato effort and we should help out where we are needed, we are one of the few fighting forces in Afghanistan and with that comes responsibility.
"In any case, we are unlikely to be sleeping soundly in Lashkar Gar [the capital of Helmand] if security is unravelling next door in Kandahar. But this, if it happens, should be at the end of a successful year in Helmand. We should leave Helmand with our head held high – we do not want a Basra scenario."
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