Russia has agreed to co-operate on Nato's programme to defend against ballistic missile attacks, Nato's chief has said.
Anders Fogh Rasmussen said at a Nato summit in Lisbon that the two sides had agreed in writing that they no longer posed a threat to one another.
"For the first time the two sides will be co-operating to defend themselves," Mr Rasmussen said.
The Lisbon summit has been redrawing Nato's focus to face new challenges.
Russian President Dmitry Medvedev said of the summit: "A period of very difficult, tense relations has been overcome."
This is the first Nato summit Russia has attended since the Russia-Georgia war two years ago.
Nato members had earlier agreed on a programme to develop and deploy defences against ballistic missile attack on their territories.
Mr Rasmussen said he had extended an offer to Russia to co-operate on the programme and was "very pleased that [Russian President Dmitry] Medvedev has taken up that offer".
Mr Rasmussen said this agreement was of "real political importance" and a "true turning point".
There would be an exchange of information on the threats to European skies, he said, and the two sides "could conceivably co-operate on shooting down an incoming missile".
Mr Rasmussen said: "The Nato nations and Russia have today agreed in writing that while we face many security challenges, we pose no threat to each other."
He said Russia had agreed to allow more supplies to travel through Russian territory to support Nato's mission in Afghanistan and to allow equipment out as well.
Moscow withdrew from Afghanistan in 1989 after a bloody 10-year conflict.
Mr Rasmussen said there would also be increased co-operation with Russia on terrorism, weapons of mass destruction, piracy and counter-narcotics.
Mr Medvedev hailed the "constructive atmosphere" of the talks, adding: "We have ambitious plans, we will work across all directions, including European missile defence and the Russia-Nato council has demonstrated that."
Mr Medvedev praised US President Barack Obama for his "courageous" rejection of the version of the European missile shield projected by former President George W Bush.
But Mr Medvedev said many details of the shield plan were still uncertain and that the scheme would "only be peaceful when it is universal".
And he warned: "Our participation has to be a full-fledged exchange of information, or we won't take part at all."
Mr Obama hailed the "resetting" of Nato-Russia ties.
"We have agreed to co-operate on missile defence - we have turned a source of past tension into a source of co-operation," he said.
The missile shield programme was a "clear plan to protect" allies in Europe, Mr Obama said.
He also again appealed to the US Senate to ratify a new Start treaty he has agreed with Mr Medvedev.
The treaty would reduce both countries' nuclear arsenals and allow each to inspect the other's facilities.
Mr Obama said the Senate should "rise above partisanship" to ratify the deal.
He said there had already been "18 hearings and nearly 1,000 questions answered" on the treaty and it would have "to start over from scratch in January" if it were not ratified.
Mr Obama said he had won "overwhelming support" among Nato allies for the Start deal.
Mr Medvedev also called on the Senate to be "responsible" and ratify the deal.
The two-day Lisbon summit has been billed as one of the most important in Nato's history, as it seeks to update its strategy and structure to face new security threats.
On Friday member states agreed a new 10-year "strategic concept", a document that defines the fundamental nature of Nato's role in the world .