Nato leaders are today expected to sign up to an agreement to construct a defensive shield for Europe against a ballistic missile attack.
Prime Minister David Cameron will join other alliance leaders for a summit in Lisbon which is expected to see for the first time a political agreement on a missile defence system for European territory.
It will see the commitment of US resources to the defence of Europe while inserting a Nato command-and-control element into the system.
While much of the focus in the run-up to the summit has been on Afghanistan, the first day of the gathering will concentrate on other matters - most notably missile defence and Nato reform.
Despite reported last-minute concerns by Turkey, British officials are confident that the agreement on missile defence will go ahead.
There is even an expectation that there will be discussion of co-operation with Moscow on the system when Russian President Dmitri Medvedev joins the alliance leaders in the Portuguese capital tomorrow for a Nato-Russia.
The Russians have largely dropped their previous objections after US President Barack Obama announced that he was abandoning plans by the Bush administration to site the interceptor missiles and radar in Moscow's "backyard" in Poland and the Czech Republic.
The intention now is that - initially at least - the main "hardware" will be carried on US warships stationed in the Mediterranean, although the possibility of land-based facilities has not been ruled out.
The system will be developed over a 10-year period, with Britain contributing the early warning radar station at Fylingdales and the data processing centre at Menwith Hill - both of which are part of the existing US missile defence system - to the programme.
The system was being described in Whitehall as an "insurance policy" for the future, as more countries develop ballistic missile technology.
It is also seen as a firm indication of America's continuing commitment to the defence of Europe - particularly in economically straitened times.
European missile defence is one of the key elements of the new Nato "strategic concept", expected to be adopted by the summit, which will guide alliance policies for the next decade.
It includes the commitment of new resources against the threat of cyber-attack and reform of Nato's command-and-control structure, cutting the number of personnel by around a third from 13,000 to 9,000.
The main discussions on Afghanistan are expected to take place tomorrow when the Nato leaders will be joined by the leaders of the other troop contributing nations as well as President Hamid Karzai and United Nations Secretary-General Ban Ki-moon.
The meeting is expected to agree a framework for the transition of responsibility for security to the Afghans, with the initial transfers beginning next year and the whole process to be completed by 2014.
Mr Karzai recently provoked anger in alliance capitals when he demanded international forces reduced their military operations and halt their night raids against suspected Taliban leaders - seen as one of the coalition's most successful tactics.
Nato's civilian representative in Afghanistan, Mark Sedwill, said the Afghan president's comments were "not helpful".
"We have different perspectives, that's natural. It is much better if we work those different perspectives out in private," he said.
Nato Secretary-General Anders Fogh Rasmussen insisted that, despite earlier setbacks, the coalition mission in Afghanistan was back on the right track and he was "very optimistic" for the future.
"I think that, seen retrospectively, we under-estimated the challenge and our operation in Afghanistan didn't have sufficient resources, and yes, that was a mistake," he said.
"We're on the right track now and that's why I'm very optimistic about our Afghanistan operation and we'll make a positive announcement in Lisbon - that the handover is about to begin."
Speaking on BBC Radio 4's Today programme, Mr Rasmussen said the pullout by 2014 is a "realistic" road map, but added that the process is conditional on improvements within Afghan security forces.
The Nato secretary-general said: "Our mission will end when the Afghans are capable to take responsibility themselves.
"We hope that they will be able to take such lead responsibility all over Afghanistan by the end of 2014.
"But obviously this process must be condition-based. We have to make sure that the Afghans are actually capable to take responsibility before we hand over responsibility to the Afghans."
He added that all Nato allies would "stay committed as long as it takes".
Defence Secretary Liam Fox said the missile system would cost Britain about "something like £2 million a year" over the next 10 years.
"We think it's a good thing to have a missile defence system which is Nato-based," he told BBC Radio 4's Today programme.
"We think that provides us with communal protection over the years ahead, it's cost-effective for us, and there are some 30 countries now which either have or are developing ballistic missiles that this will give us protection from."
Dr Fox said the Nato summit would be "workmanlike, but extremely important for the future of Nato".
On Afghanistan, Dr Fox added that the Government does not know how many troops would need to remain in the country in non-combat roles after 2015.
The Defence Secretary said it is "unlikely" that all of the 10,000 troops currently there will be needed after 2015 in training or mentoring roles.
But he was unable to say whether the Government would keep "large numbers" there.
"Well, we may or may not, it will depend as we go through this as a coalition, that's one of the things we will be discussing over the next two days, exactly how we see the shape of transition happening," he said.
He added: "We don't know what numbers will stay. We certainly don't want to be in a combat role."
Dr Fox told Today that even after 2014 - when Afghan security forces are supposed to be in charge - the Kabul government would require "help and advice".
"If you look at what is happening in Iraq there are still high levels of violence there, we are not going to suddenly transition into a safe, secure and peaceful Afghanistan overnight," he said.
"It's going to take a long time. We are going to have to continue with the aid programmes, we are going to have to continue with better governance, these are all required for long-term stability.
"Gradually it will be less of a military role and more of a civilian role and for the NGOs and the aid agencies, they are going to be in Afghanistan for a long time."Reuse content