Lisbon, Portugal (CNN) -- The NATO meeting in Lisbon ended on a high note Saturday, with U.S. President Barack Obama calling the summit "extremely productive."
Obama said that after a period in which U.S.-Europe relations were "severely strained, that strain no longer exists."
"We came to Lisbon with a clear task, to revitalize our alliance," he said, explaining that the alliance has resulted in a unified Europe, a strong ally of the United States, and prosperity for the United States, Europe and the world.
NATO, which held a two-day summit, adopted a new strategic concept that will serve as the alliance's road map for the next ten years and reconfirmed "its commitment to defend one another against attack as the cornerstone of Euro-Atlantic security."
"NATO leaders reiterated their commitment to ensure that the Euro-Atlantic Alliance has the full range of capabilities necessary to deter and defend against any threat to the safety and security of the populations of member countries," the alliance said in a press release rounding up highlights of its meeting.
"To this end, they decided to develop the capability to defend European territory and populations against missile attack as a core element of collective defence and to extend an offer to Russia to cooperate with NATO in this regard," the release said.
The alliance also focused on Afghan security, missile defense, modernizing, a "fresh start in relations with Russia" and streamlining its military command structure.
NATO's leaders agreed to forge a "capability to protect NATO's populations and territories in Europe against ballistic missile attacks," according to the release.
Alliance leaders said "they see this as a core element of NATO's collective defence task in view of the growing threat of the proliferation of ballistic missile technology and weapons of mass destruction."
The 28 NATO countries were joined by others, including the 20 partners who are contributing forces to the mission in Afghanistan, representatives of the United Nations, the World Bank and the European Union, and Afghan President Hamid Karzai.
"Together, they launched the process by which Afghan security forces will increasingly take the lead for security operations across the country, starting early 2011," NATO said.
NATO Secretary General Anders Fogh Rasmussen signed a long-term partnership agreement with Karzai under which "NATO will stay as long as necessary to support Afghanistan until it can no longer become a safe haven for terrorism."
As for Russia, NATO allies agreed with Russia "to jointly expand support for Afghanistan, including by broadening transit arrangements, extending training of counter narcotics officials and providing equipment to Afghan security forces."
Obama and Rasmussen announced that NATO heads of state will gather again in 2012, in the United States.