President Barack Obama won Nato summit agreement to build a missile shield over Europe, an ambitious commitment to protect against possible Iranian attack while demonstrating the alliance's continuing relevance — but at the risk of further aggravating Russia.
On another major issue, Mr Obama and the allies are expected to announce plans on Saturday to begin handing off security responsibility in Afghanistan to local forces in 2011 and to complete the transition by the end of 2014.
That end date is three years beyond the time that Mr Obama had said he would start withdrawing US troops.
While celebrating Friday's missile shield decision, Mr Obama also made a renewed pitch for Senate ratification back in the US of a nuclear arms treaty with Russia, asserting that Europeans believe rejection of the deal would hurt their security and damage relations with the Russians.
Two key unanswered questions about the missile shield- will it work and can the Europeans afford it? — were put aside for the present in the interest of celebrating the agreement as a boost for Nato solidarity.
"It offers a role for all of our allies," Mr Obama told reporters on Friday. "It responds to the threats of our times. It shows our determination to protect our citizens from the threat of ballistic missiles."
He, however, did not mention Iran by name, acceding to the wishes of Nato member Turkey, which had threatened to block the deal if its neighbor was singled out.
Under the arrangement, a limited system of US anti-missile interceptors and radars, already planned for Europe, will be expanded to include interceptors in Romania and Poland and possibly a radar in Turkey. This would create a broad system that protects every Nato country against medium-range missile attack.
Nato plans to invite Russia to join the missile shield effort, although Moscow would not be given joint control. The gesture would mark a historic milestone for the alliance, created after World War II to defend Western Europe against the threat of an invasion by Soviet forces.
As for the US-Russia arms treaty, Mr Obama was backed by Nato secretary general Anders Fogh Rasmussen of Denmark, who told reporters that the treaty, called New start and signed last April by Mr Obama and Russian President Dmitry Medvedev, would improve security not only in Europe but beyond.
"I would strongly regret if it is delayed," Mr Fogh Rasmussen said. "A delay would be damaging for security in Europe, and I urge all parties involved to ratify it."
Mr Obama needs 67 votes in the Senate for ratification, and many Republicans have balked at even taking a vote before the new, more heavily GOP Congress convenes in January.