As Afghanistan goes, so goes NATO. Interviews across the U.S foreign policy establishment reveal a unified belief that the authority of the transatlantic alliance will be won or lost in the Afghan war.

Military issues rarely transcend ideological divisions within the U.S. diplomatic class. Yet stateside, it's now more than rhetoric to define Afghanistan as NATO's existential test. There is an emerging U.S. consensus that if Europe does not reverse itself and significantly reinvest in the war effort, the transatlantic military treaty will cease to matter.

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"NATO's credibility is on the line," said Sandy Berger, who served as national security advisor during the Clinton administration. "NATO needs to succeed in Afghanistan," Berger added in an interview. "If it doesn't, it really does undermine the vitality of the alliance."

Or as John Bolton put it, "Ironically, the risk here is that Afghanistan looked like the future of NATO. It could become its graveyard." A former U.S. ambassador to the United Nations for George W. Bush, Bolton added that, "It's in our interest to keep NATO viable. But it's not in our interest to keep NATO viable at any cost."

President Obama closes his first week abroad at the NATO summit on Friday and Saturday, marking the 60th anniversary of the alliance. But behind the celebrations are deep divisions between member-states over NATO's war in Afghanistan and the role of the alliance itself.

Last Friday, Obama pledged still more U.S. boots on the ground in Afghanistan. The president also committed thousands of U.S. civilians over the long term to Afghani infrastructure, and by consequence the U.S. to nation building. Meanwhile, some of the closest U.S. allies are gradually retreating from the war. Britain began its staggered troop withdrawal from Afghanistan this week.

The divergent allied actions have forced NATO to face a crucial question: is the military alliance soon to be nothing more than a diplomatic pact?

After the United States was attacked on September 11, 2001, NATO invoked Article 5, its mutual defense declaration, for the first time. But now the endurance of Article 5 is being tested and as the fulcrum of the alliance, so is NATO.

Afghanistan amounts to NATO's first war abroad, its first land war and its first trial in the post-September 11 era -- an era where threats are less from states than from rogue actors.

"This attempt to reposition NATO to a robust expeditionary force, sort of the UN with guns, has been more of a detriment than help," said Max Boot, a foreign policy advisor to Republican John McCain during the 2008 campaign and a senior fellow at the Council on Foreign Relations. "It was a noble experiment but it didn't work out.

"Afghanistan is becoming a coalition of the willing by another name," Boot continued. "We've seen the crack in [NATO's] edifice exposed."

As NATO expert Elizabeth Sherwood-Randall, Obama's recently hired European affairs advisor, reportedly said before taking the post: "The glue of this alliance is gone."

The White House has avoided the same blunt declarations. The Obama administration continues to hold out for a significant escalation of European resources in the region. Yet of late, save commitments by outliers like the Netherlands, U.S. greybeards are not optimistic that Europe will stay significantly in the fight.

NATO's military alliance certainly may withstand even a full on European retreat from Afghanistan. Since its formation in 1949, NATO has been no stranger to existential crises. From the 1956 Suez Canal crisis, to the fall of the Soviet Union and most recently to the war in Iraq, the preeminent transatlantic treaty has endured.

Yet particularly after the Soviet Union imploded, what Henry Kissinger called the "core institution" of the Cold War became a defensive alliance without an antagonist. The coalition formed to "keep the Russians out, the Germans down and the Americans in" wanted the Russians in, the Germans up and ever-more democracies in. The terrorist attacks of 2001 seemed however to clarify NATO's role and briefly offer the alliance renewed raison d'etre.

That glue has already faded. Europeans lack the sustained will in Afghanistan, for no reason more than the 9-11 attack was on U.S. soil.

And even amid some good news for NATO -- after withdrawing from NATO's military command structure in 1966, France recently rejoined its ranks -- the war's worsening terms has expended what little public support existed across Europe to risk its troop's lives.

International soldiers' deaths in the Afghan war rose by 37 per cent in 2008, six fold the rate of Afghan troops. Nearly nine of ten allied soldiers in Afghanistan remain under the command of the NATO, for now at least.

The U.S. musters slightly more than half of the roughly 70,000 international troops in Afghanistan today. But this year Obama committed some 21,000 additional soldiers, and counting. By 2010 the United States will constitute about two-thirds of NATO's commitment in Afghanistan even as the troop commitment by other allies, like Canada, expires in 2011.

NATO has then come toward its sixth decade facing an identity crisis, testing the reach of its ideals and the elasticity of its original intent.

The summit this week will, perhaps unintentionally, expose that conflict between past and future missions. The agenda is consumed with Afghanistan. But the summit is occurring at the Rhineland border cities of Kehl and Strasbourg, once the fault line of the great wars of Europe. The alliance meant to in part help keep the peace between Germany and France is now caught in a war far beyond Europe's borders.

Some U.S. foreign policy figures, like Boot, believe that NATO will remain a vital democratic alliance regardless of the outcome in Afghanistan.

The reach of that democratic alliance though is also divisive. The ambitions of Ukraine and Georgia to join NATO have wedged the alliance from Russia, a nation NATO now aims to embrace. Yet that debate as well has taken a back seat to the Afghan war, especially as the war's outcome appears increasingly opaque.

Concern is rising in U.S. diplomatic circles over whether Afghanistan could turn into Obama's Vietnam. Zbigniew Brzezinski, Jimmy Carter's National Security Advisor, calls Afghanistan NATO's "critical test."

But Brzezinski's chief concern is quickly shifting from joint-commitments to the potential for over-commitment. If the United States and NATO get "bogged down" in Afghanistan and Pakistan, he said in an interview, it could be "a bottomless pit."

Similar issues came to the fore during NATO's 50th anniversary as well. In 1999, the alliance celebrated its inception also rife with divisions over an ongoing war, then in Kosovo. And like Afghanistan today, the alliance was haunted by the potential for Kosovo to turn into a quagmire.

"That summit was to hold the alliance together," recalled Berger, who helped organize the 1999 conference for the Clinton White House. "There is a question again of NATO solidarity" and in Berger's words a new question as well: will Europeans offer "serious participation" in Afghanistan "lest this become an American war?"

Yet with the financial crisis still smothering the diplomatic agenda, it remains unclear how much political capital Obama will spend in the coming days to publicly pressure nations like Germany and France to significantly join in a war that remains, perhaps soon by treaty alone, Europe's war as well.

David Paul Kuhn covers national affairs for RealClearPolitics and is the author of The Neglected Voter.