Nato is repositioning itself for a future as a key force in the "war on terror", according to its US ambassador, Nicholas Burns.
Nato's troops can now expect to be deployed anywhere in the world
The organisation has been struggling to define its role in the world since the collapse of the Soviet Union, the threat from which was the basis of its foundation.
But Mr Burns told BBC World Service's The Interview programme that the threat to Nato's members was now global - so the alliance had to be reshaped to respond.
"Nato's primary purpose right now is to defend Europe and North America by going well outside of Europe and America to do that," he said.
"The great threats to Britons, and to French citizens and Americans, certainly come from the juxtaposition of global terrorism with chemical, biological and nuclear suitcase bombs.
"That threat exists. It exists in the Middle East, and it exists in Central Asia."
Questions have been asked about Nato's role in the world since the collapse of the Soviet Union in 1991.
Although the alliance intervened in Kosovo, it did not formally operate in the military campaigns in either Iraq or Afghanistan.
But Mr Burns emphasised the role the organisation was currently playing:
The rapid response force is seen as key part of the "war on terror"
"Nato has just gone into Central Asia to take over the Afghan peacekeeping force, which is a very good thing," he said.
"The great majority of Nato countries have soldiers on the ground in Iraq.
"Nato's purpose is to be on the front lines, to fight the war on terrorism for Europeans, and for Americans and Canadians.
"That's where we've turned the organisation, that's how we've transformed it over the last two years."
In particular, Mr Burns stressed the role the newly-established rapid response force would play.
The force is designed to be able to deploy within days to anywhere in the world if needed, with between 20,000 and 30,000 troops.
Mr Burns refuted suggestions that it would merely be a tool of American foreign policy.
"If there were a terrorist threat to any one of us - European countries or North American countries - the response force could be activated to meet it, but 26 countries will decide that," he said.
"If there were a hostage situation - of course, there have been a lot of them over the last 15-20 years - the response force could be used for that.
"A humanitarian intervention in Africa or the Middle East or Central Asia would also be subject to this. There are a variety of ways to use it, but it's not some kind of unilateral force determined by the United States."
Mr Burns conceded that the divisions between Nato countries in the build-up to the war in Iraq had nearly been the end of the organisation.
But he said the unity had come back between the countries since the crisis, which began with Turkey's request for help, fearing it would be attacked by Iraq.
Mr Burns said that while most countries had been keen to respond, a "minority" - France, Germany and Belgium - had resisted, prompting a major rift within the alliance.
Nato is now in control of peacekeeping in Afghanistan
"It was, I think, one of the greatest crises in Nato's history.
"It certainly was a near-death experience, as I and others said at the time.
"Having gone through that experience, I think many countries are now more confirmed that we've got to stick together."
Some US commentators have suggested that Nato's role in combat operations is likely to be massively diminished, however, as the multilateral structure prevents decisions being made quickly.
This, some argue, frustrates those in the White House, who instead will peruse a policy of forming a coalition around their mission.
But Mr Burns said that, though Nato had been left out of the main fighting in Iraq, it would continue as a key element of military campaigns around the world - on behalf of all its members.
"You don't hear [Secretary of State] Colin Powell or [Defence Secretary] Donald Rumsfeld saying Nato is finished as a war-fighting machine," he argued.
"I think it's the American unilateralists - those people largely outside our government - who are arguing that the United States is somehow so superior in military strength that it can somehow afford to go it alone.
"Those people are mistaken."