Britain and the United States must be ready to send troops back into Afghanistan if a withdrawal deal with the Taliban - set to be signed on Saturday - unravels and the militant force attempts to "march on Kabul", a former British military chief has warned.If it succeeds, General Lord Richards said the agreement would represent a draw between the two sides following almost 19 years of war."My feeling is that we don't certainly have to be ashamed [of] this if it will work, but there is a lot of risk," Lord Richards told Sky News in an interview."We must be prepared to do again with the Americans and our allies what might be required for another few years to remind the Taliban that there is a line beyond which we won't go."Image:The general says we must be prepared to go back 'for another few years' if the Taliban deal faltersAsked if he meant that Britain and it allies must be ready to send forces back into the country, the general said: "Yes - if this deal doesn't work and the Taliban start marching on Kabul for example then we have got to help our Afghan allies and make sure the vast majority of Afghans - who don't want that - are not ruled by the Taliban."That then would be a tragic defeat."The comments came as the United States and its once-sworn enemy, the Taliban, prepared to sign the deal in the Qatari capital of Doha.President Donald Trump said Mike Pompeo, his secretary of state, would be present.At the same time, defence secretary Mike Esper will travel to Kabul to sign a separate declaration with the Afghan government, which has not been directly involved in the US-Taliban negotiations.The Taliban agreement will see a reduction in US, British and other NATO-led forces in Afghanistan but not a complete withdrawal straight away.US-Taliban deal raises hope for peaceIn return, the Taliban is expected agree to open formal dialogue with Afghanistan's fractious political parties, which are still feuding over the results of a general election from last year.The aim will be to agree a permanent ceasefire across the country and to set the conditions for some sort of power-sharing arrangement.The Taliban must also cut all ties with al Qaeda and other militant groups."If the Taliban and the government of Afghanistan live up to these commitments, we will have a powerful path forward to end the war in Afghanistan and bring our troops home," President Trump said in a statement."Ultimately it will be up to the people of Afghanistan to work out their future.

We, therefore, urge the Afghan people to seize this opportunity for peace and a new future for their country."But there are concerns that the Taliban will renege on its commitments, including to cut ties with al Qaeda.A further drawdown of foreign troops will hinge on how this next phase goes.

The Taliban - which has been framing the deal as a victory - wants all foreign forces out of the country.The prospect of a Taliban revival is a far cry from the goals of US and British forces when they invaded Afghanistan in 2001 following the September 11 attacks on the United States.The goal was to topple the Taliban, which had harboured al Qaeda and its chief, Osama Bin Laden, who plotted the airliner carnage.Image:US soldiers fighting in Kandahar province, southern Afghanistan, in June 2011The regime rapidly fell but an insurgency flared in its place.Taliban guerrilla tactics of bombings, ambushes and mortar attacks ensnared rotation after rotation of an ever-increasing footprint of US, British and other NATO-led forces.Lord Richards said opting to negotiate a deal with the enemy could still be worth the blood and treasure."If this truce works and the Taliban have seen the light of day and can be integrated in a constructive way and there's no hint of any more terrorist operations being mounted in Afghanistan, then I for one will say this was a necessary war that we fought well and the result has been a draw - if not the conventional victory we might have hoped for back in 2006," he said.After toppling the Taliban, the goals of the mission evolved.The United States and its allies said they wanted to build a democratic government in Afghanistan based around the rule of law, women's rights and good governance.

There was also an aim - led by Britain - to rid Afghanistan of its lucrative opium crop.Efforts began to build up the Afghan security forces and government ministries.But, while gains were made in areas such as education, the coalition's ambitions had to be curbed in part because of the insurgency, corruption and a lack of political will.More than 2,400 US military personnel and more than 450 British servicemen and women died in the conflict, while many more were injured.Image:NATO forces are still in the country, offering support and advice to the Afghan military and policeA far heavier price was paid by the Afghan population.Tens of thousands of security personnel have been killed since 2001 as well as tens of thousands of civilians.By the end of 2014, NATO announced an end of its combat operations in Afghanistan, with a significant reduction in troops.But US, British and other countries' special forces continued to work with their Afghan partners on raids against the Taliban, al Qaeda, as well as an emerging presence of Islamic State-linked fighters.At the same time a NATO training mission evolved to offer support and advice to the Afghan military and police.The US is by far the largest contributor with some 13,000 personnel on the ground.

Britain has about 1,100 servicemen and women in the country.Saturday's deal is expected to see these numbers fall, with the United States set to reduce its footprint to 8,600 troops.

The move could well provide a boost to President Trump in the run up to the US presidential elections.The anticipated signing ceremony comes after both sides agreed to a week-long reduction in violence across Afghanistan that began last Saturday as a trust-building exercise.