It is therefore vital to the UK that Afghanistan becomes a stable and secure state that is able to suppress violent extremism within its borders.
The Taliban gave safe haven to Al-Qaeda, which allowed terrorists to plan and carry out terrorist attacks around the world, including the atrocities of 11 September 2001.
That is why the UN authorised a NATO-led military intervention.
British Forces are operating in Afghanistan as part of that NATO intervention to prevent Afghan territory from again being used by Al-Qaeda as a base from which to plan attacks on the UK and our allies.
Afghanistan is not yet strong enough to look after its own security. The presence of NATO forces is preventing Al-Qaeda or the Taliban regime from returning, while the UK and other NATO forces train Afghanistan's security forces to take over the task from themselves.
In fact in some areas of Afghanistan the Afghan Forces have already grown sufficiently in capacity and capability to take over responsibility for security from NATO forces.
One such area is the District Centre of Lashkar Gah, the capital of Helmand province where the vast majority of British troops in Afghanistan are based. Afghan National Security Forces took over the lead responsibility for security here from British troops on 20 July 2011. Today, no coalition troops are involved in the day-to-day security of the town.
It is under NATO's International Security Assistance Force that the British Armed Forces, along with the militaries of over 48 other nations, are deployed to Afghanistan.
Working as part of this international community, the UK is supporting the Afghan government as it builds a peaceful and resilient nation, shaped by the will of the Afghan people.
We do not seek a perfect Afghanistan, but a stable Afghanistan able to maintain its own security and prevent Al-Qaeda from returning.
Right now British Forces are taking the fight to the Taliban in southern Afghanistan, facing down the insurgency, protecting the Afghan people and laying the conditions that will contribute to stabilisation efforts.
They are also providing direct military support to the Afghan National Security Forces including the Army and Police through training and mentoring, building up those institutions so they can take on the job of confronting the insurgency themselves.
Twenty years of war completely destroyed Afghanistan's infrastructure, economy and institutions.
UK forces are therefore also working alongside civilians in multi-national Provincial Reconstruction Teams to help the Afghan State increase in size and capacity so that it can protect its people, giving them physical and economic security.
The military intervention in Afghanistan is just one aspect of the UK's involvement in supporting the country.
The UK is working relentlessly on all fronts to help the Afghan Government improve governance, both nationally and locally, tackle corruption and the drugs trade, build up the rule of law, promote economic development - investing in infrastructure, legal alternatives to poppy, jobs - and improve access to education and healthcare.
The UK remains committed to these things. We are not imposing a Western model, but are working alongside 60 countries to give the people of Afghanistan a stake in their future.
It is a hard job. Progress is slow, but the progress we have made is real, and we are working towards 2015 when it is expected that foreign troops are no longer needed on the frontline and Afghanistan can stand up to terrorism itself and offer its people hope for the future.
The UK Government's decision that UK troops will no longer be in a combat role or be in Afghanistan in significant numbers by 2015 reflects Afghanistan's President Karzai's goal that by 2014, Afghanistan's own Security Forces should be responsible for security.
Walking away now would not solve the problem but compound it. It would risk recreating the chaos in which terrorism could re-establish itself.
Failure would deal a serious blow to the credibility of the UK, NATO and the international system, would hand victory to violent extremism with all that would entail, and would damage regional stability.
International support for Afghanistan will not end in 2014. We have a vital national security interest in preventing Afghanistan from once again becoming a safe haven for international terrorism. We and our allies will go on having a strong relationship with Afghanistan based on diplomacy, trade, aid and development. We shall also continue to support the development of the Afghan National Security Forces.