Catherine Field: Why are we here? Nato stops dodging the hardest question

The leaders of North America and Europe gather in Lisbon today for a two-day summit aimed at resolving their military alliance's vexed future and the growing friction in their economic ties.

Nato's expeditionary doctrine, Afghanistan, nuclear deterrence and relations with Russia will dominate the 28-nation North Atlantic Treaty Organisation meeting in the Portuguese capital.

It will be followed by United States President Barack Obama's first meeting with the heads of the European Union, where tense exchanges are possible over widening discrepancies in fiscal and monetary policies.

The Nato summit will look at the question that the Western alliance has dodged for nearly 20 years - what is the organisation for?

Nato, created in 1949 in the deepest chill of Europe's Cold War, was described by its first secretary-general, Lord Ismay, as designed to "keep the Russians out, the Americans in and the Germans down".

Since German reunification in 1990 and the fall of the Soviet Union in 1991, two of these objectives have become almost irrelevant - the threat from Moscow has receded and Germany is accepted as a mature democracy - and a US withdrawal from Europe seems unthinkable.

"If asked to respond to the question of Nato's primary purpose and identity in 2010, no two alliance members would offer the same answer," say analysts Steven Pifer and Justin Vaisse at the Brookings Institution, a US think-tank.

On the table in Lisbon is the outcome of an 18-month attempt by Nato chief Anders Fogh Rasmussen to weave common strands of thinking into a "strategic concept" that leaders can endorse.

Diplomats in Brussels say the draft document is at its clearest in reaffirming the primary role of Nato as an organisation of mutual assistance against aggression.

Under the charter's article five, members agree that an armed attack against any of them "shall be considered an attack against them all."

It will also identify emerging threats against which the alliance should stand firm.

These include terrorism, acquisition of nuclear weapons and missiles by rogue states and piracy.

Beyond that, though, the text gets fuzzier as it goes into fierce areas of discord. It includes the language to describe the role of nuclear weapons in Nato, budgetary commitments - an issue that has produced periodic bursts of concern in Washington at the decline in its allies' defence spending - and whether the alliance should embark on missions beyond its border.

The notion of an "expeditionary" role for Nato, under which it intervened in hotspots or failed states that posed a security threat, gained strength in the 1990s, when the alliance intervened in Bosnia-Hercegovina and Kosovo.

But enthusiasm for the concept has evaporated because of the problems of the war in Afghanistan, now nine years old. Since the start of this year, 645 Nato troops have been killed there, 124 more than in the whole of last year.

The Lisbon summit is unlikely to hear demands for more troops in Afghanistan, although alliance members will be pressed to contribute trainers to help enable the Afghan army to take over "primary security responsibilities" in 2014.

The Afghan quagmire has crystallised the argument - espoused especially by France and Germany - that Nato should evolve into more of a political organisation, say sources.

How far this idea runs will get an early test, for President Dmitry Medvedev is to join the summit to hear proposals that Russia join a missile shield to defend Europe from ballistic missiles fired from the Middle East.

"I think we are witnessing a fresh start in the relationship between Nato and Russia and maybe I could go further and say a fresh start in the relationship between Russia and the West," Rasmussen said.

Moscow had opposed the shield, seeing in it an attempt to neutralise its nuclear deterrence. Getting it to join in would build trust and open the way to widening permission to bring Afghan war supplies through Russia.

Less success seems likely in the EU-US meeting, which takes place against widening divergence on economic co-ordination.

In the firestorm that struck markets in 2008, the world's biggest economy and the world's biggest trade bloc had strategies to lower interest rates and open the money spigot to save their economies.

Since then, though, the EU countries have switched to belt-tightening, seeking with ever greater desperation to reduce the deficits in Greece, Ireland and Portugal.

America has pursued its "quantitative easing," or monetary stimulation. This incenses those in Europe who see a de facto devaluation of the US dollar and a spur to inflation. "With all due respect, US policy is clueless," said German Finance Minister Wolfgang Schaeuble, whose country is the linchpin of the eurozone, on November 4.