David Cameron has given a "firm commitment" that British combat operations in Afghanistan will come to an end by 2015.
In a statement to the Commons, the prime minister outlined the agreements made by alliance leaders at a Nato summit in Lisbon at the weekend.
Labour leader Ed Miliband said he was determined to build "unity of purpose" with the government on Afghanistan.
Cameron told MPs there were effectively three summits: one on Afghanistan, one about the future of Nato and Nato-Russia meeting.
He reaffirmed his commitment to end British involvement in combat operations in Afghanistan by 2015, telling the Commons that Britain would then have "played a huge role in the international coalition and made massive sacrifices for a better, safer and stronger Afghanistan".
On the future of Nato, Cameron said Nato agreed to develop a ballistic missile defence system for Europe. It will be in place by the end of the decade and paid for from Nato's existing resources.
The prime minister said there will be "challenges" in the relationship between Nato and Europe.
He added that Russia should withdraw troops from Georgia.
Cameron told MPs: Nato remains "the future of our bedrock defence".
Making his return to Parliament after two weeks on paternity leave, Labour leader Ed Miliband welcomed the statement, stating he was determined to build "unity of purpose" with the government on Afghanistan.
"Can I say in that context that we support the outcome of the Nato summit on Afghanistan," he said.
Miliband said Labour "strongly support" the Afghan forces taking "full security responsibility" in 2014 and agree that ending British combat operations in 2015 is "right".
The Labour leader added that it was right to try and build on relations with Russia.
Miliband questioned the prime minister on the potential "training" role for British troops in Afghanistan after 2015.
He said: "As you will know, the nature of training in Afghanistan often involves frontline exposure, so maybe you can say something more about whether troops will effectively be in some sort of fighting role beyond that date?"
In response, the prime minister said Britain has added itself "320 trainers".
"I very much see this as training and not combat. I think by that stage we will be looking at something that is much more of a training mission rather than quite as much as embedding that we've had now."
Cameron said that the transition to Afghan control would begin early next year and meet President Karzai's objective for Afghan forces to "lead and conduct security operations in all provinces by the end of 2014".
Former foreign secretary Sir Malcolm Rifkind asked about tactical nuclear weapons.
The prime minister said the problem is that relations between Russia and Nato have been difficult over years. However he added they are "thawing".
Former Labour defence secretary Bob Ainsworth asked the prime minister why he was so particular about the deadline for British withdrawal.
In response, Cameron said the British people have the right to know when the troops will come home.
The alternative to a single, far-off deadline would be a series of mini-deadlines, that would be unhelpful, he said.
He told MPs: "I am confident we will succeed in our goals by 2014 and that will enable an end to combat operations and much lower numbers in 2015. But I wanted to make it clear to people so that they can see an end point."
Former conservative public accounts committee chair Edward Leigh asked whether the deadline could "incentivise" the Taliban to step up their campaign.


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