Rasmussen told a news conference the reforms envisioned in the strategic concept showed that NATO had "learned the lessons" of the Afghan conflict by committing to better coordinate military and civilian resources in the future, improve defense planning, and maintain "robust, mobile and deployable" forces for far-flung counter-insurgency operations.

NATO's Secretary General Anders Fogh Rasmussen, U.S. President Barack Obama and British Prime Minister David Cameron (L to R) talk before posing for a group photo at the venue of the NATO summit in Lisbon, capital of Portugal, Nov. 19, 2010. [Wang Qingqin/Xinhua]

But he acknowledged concern about declining European military spending and the allies' doubts about further Afghanistan-type operations are leading many Americans to question the enduring value of the transatlantic link.

Obama sought to allay those concerns, calling the Atlantic Alliance the "cornerstone" of U.S. foreign policy. "The new Strategic Concept that we are embracing shows that NATO is fully united about the way forward and committing to addressing the full range of security challenges of this century," he told reporters in Lisbon.

"The strong ties between the United States and Europe will continue to grow for years to come," he said.

The U.S. president gave particular prominence to the commitment to missile defense after years of procrastination among the allies. The deal was reached after a last-minute deal between France and Germany on how the system will fit into NATO's nuclear stance, and after the United States had ceded to Turkish demands that the text would not specifically identify Iran's ballistic missile program as the target of the new system.

Germany wanted to tie the building of the anti-missile defenses to a commitment for more nuclear disarmament. France, however, was wary of any language that would undermine its nuclear strike force.

In the end, the NATO leaders said the alliance is committed to arms control and would seek to "create the conditions for a world without nuclear weapons."

However, they also said that as long as nuclear weapons existed elsewhere, an "appropriate mix of nuclear and conventional capabilities" would remain a "core element" of NATO's strategy.

The missile defense deal aims to link ongoing work by NATO nations to bring their individual early warning and interceptor systems together into a single network designed to protect troops and military installations against a short- and medium-range ballistic missile attack. That system will then be integrated into a broader U.S. system against long-range missiles to provide territorial defense for the alliance nations.

At a time of shrinking defense budgets, NATO says the costs will be minimal: 200 million euros (around 274 million U.S. dollars ) over 10 years shared among all the allies, in addition to the 800 million euros (1.096 billion dollars) already earmarked for the shorter-range system.

Russia was vehemently opposed to a first version of the defense shield put forward by the Bush administration which would have placed a high-tech radar base in the Czech Republic and interceptor missiles in Poland.

Obama has modified that plan in an effort to win Russia over, and has assured Moscow that the target of the proposed NATO shield is not Russia's nuclear arsenal, but the threat of an attack from Iran or elsewhere in the Middle East.

The strategic concept re-affirms NATO's central mutual defense clause, insisting that they will all unite should any allies come under attack.

NATO also seeks to strengthen the partnership between Russia, the European Union, and any nations or relevant organizations across the globe that "share" its interests in "peaceful international relations."