With its focus shifting to Asia, the United States will seek a less dominant role in NATO in the future but will still turn to European allies "when the chips are down," experts say.

Washington's emphasis on the Asia-Pacific region does not render NATO irrelevant but the change signals a new era in which European states will have to be ready to manage security problems in their neighborhood without expecting the American military to lead the way as in the past.

As NATO gathers for a summit Sunday and Monday in Chicago, the message from Washington amounts to "don't count on us for everything," said Barry Pavel, a former senior official at the Pentagon and White House.

For challenges that do not constitute a threat to all alliance members, "the Bosnias, the Kosovos, the US is not going to be there all the time," Pavel, now at the Atlantic Council think tank, told AFP.

NATO's air war in Libya last year, in which Europeans led the operation with support from the United States as well as non-NATO Gulf states, was an example of how the alliance could evolve.

But for the model to succeed, NATO's European members will have to invest in aircraft, weapons and training, analysts say.

The Libya intervention exposed serious shortcomings in Europe's military power and a transatlantic gap in capabilities, with Washington far ahead of its partners.

In one of his last speeches as US defense secretary, Robert Gates issued a stark warning that if European governments failed to heed the lessons of the Libya operation and invest in their armed forces, NATO faced "a dim, if not dismal future."

Gates, a veteran of the Cold War era, said that without a change in current trends, the next generation of American leaders might not consider it worth investing in the NATO alliance.

Although the Pentagon plans to withdraw two Army brigades from Europe, the United States looks firmly committed to NATO, with a lead role in missile defense for Europe, counter-piracy naval operations off the Horn of Africa and an alliance-led war in Afghanistan.

By stressing the importance of China's rise, the United States hopes to nudge its NATO partners to pay more attention to Asia and forge security ties with countries that have strong links to Washington, including Japan, South Korea and Australia, experts say.

Despite the much-publicized US shift to Asia, the looming crisis over Iran's nuclear program remains a top priority on both sides of the Atlantic, and could well draw NATO into diplomatic and military action.

If Iran launched a ballistic missile, the launch might be picked up by NATO's new missile defense radars in Turkey or alliance radars on naval ships in the Mediterranean, according to a recent paper co-authored by Pavel.

"Thus, a single Iranian ballistic missile in a Gulf crisis will lead almost automatically to Article 5 consultations, and in some cases operations, by the alliance," wrote Pavel, referring to NATO's charter that treats an attack on one member as a threat to all.

For peacekeeping, humanitarian assistance and military action as a last resort, NATO remains the world's "best first responder," said Nicholas Burns, a former senior American diplomat who once served as US ambassador to the alliance.

Burns recalled when America tried to forge ahead without its partners on the other side of the Atlantic under ex-president George W. Bush, with disastrous consequences.

Unilateralists "were the people who said, particularly after 9/11, it's our way or the highway. And you know, if you're not with us, you're against us and we'll go it alone if we have to.

"But we found out what that's like, and it didn't work very well for our country," Burns said in a March speech.

While Washington's planned "pivot" to Asia has unsettled some in the alliance, Europe is America's biggest trading partner by far and represents "the largest collection of American allies in the world," he said.

"Europe still matters, and it is a place of vital importance to the United States, and NATO remains our vital institution," said Burns.

"When the chips are down, the Europeans are with us, and I found that time and time again in my career."