Jonathan Eyal, Europe Correspondent

Will Nato rise to new Russian challenge?

Annexation of Ukraine's Crimean peninsula testing alliance but also giving it fresh impetus

Military alliances are similar to prenuptial agreements: countries join military pacts with enthusiasm or lust but not necessarily love, don't expect these arrangements to last a lifetime, and are careful to keep count not only of the benefits, but also of the liabilities.

Not so with the North Atlantic Treaty Organisation (Nato), the United States-led military alliance in Europe which was established soon after the end of World War II for the sole purpose of defending Europe from the Soviet Union. That opponent no longer exists, the world has changed beyond recognition yet Nato, edging towards celebrating its 70th birthday, is still going strong.

So, when Nato heads of states and governments gather later this week in Britain for their summit, their main subject of discussion will not be how to extricate themselves from their old marriage vows, but precisely the opposite: how to inject greater vigour into their partnership.

The history of Nato's endurance as the biggest military alliance in the world is extraordinary. Nevertheless, past performance is not necessarily a guide to the future; Nato's most stringent tests are yet to come.

Nato's biggest attraction is that it is the only organisation in whose defence the US remains legally pledged to go to war.

Legions of academics have debated whether this guarantee, enshrined in the founding Washington Treaty of 1949, is automatic. Still, the fact remains that, on paper, Europe enjoys something Asia never did: a formal, explicit and regionwide US military pledge.

That is why the number of Nato member states kept on growing, from 12 at the alliance's foundation to 28 countries today, including former communist nations which rushed to tuck themselves under the safety of the US military umbrella.

It is important to recall that Nato's success was far from assured, and that every generation of Western politicians dreaded the possibility of the alliance's demise. During the 1950s, European leaders feared that the US would reach a separate compromise deal with the Soviet Union, effectively selling Europe into slavery.

In the 1960s, the anti-Americanism generated in Europe by the Vietnam War was confidently predicted to tear the alliance apart. A decade after that, the advent of detente was meant to produce the same result. And, when the Cold War ended in the late 1980s, politicians feared that Nato would just disappear, as the "glue" of the Soviet threat melted away.

But Nato survived all these challenges; predicting the alliance's disintegration has always been a cottage industry, yet never a profitable one.

What accounts for this unprecedented survival record, which no previous military alliance in history enjoyed? Hitching themselves to the US offered Europeans maximum protection at minimum cost. And member states broadly shared the same vision. Contrary to the official propaganda, such shared values did not necessarily include democracy: The alliance had nations ruled by military juntas as its members. But they did include a shared belief in free markets and free trade, as well as a determination to maintain the status quo in which Europe and North America were the rulers; it is always easy to agree to maintain an established order if one is running it.

Spirit of cooperation

YET probably the single biggest ingredient which accounts for Nato's longevity is not so much the organisation's capabilities, but the alliance's ability to preclude any war or military tensions between its individual European member states. It was the alliance which made old European rivalries redundant by encouraging a spirit of cooperation. And it was Nato which performed the same feat in Eastern Europe, once the Berlin Wall fell.

The incorporation of the former communist nations into the alliance infuriated Russia and may have been a contributing factor to the current Ukraine crisis, but it was the right thing to do. For without Nato membership, Poland would have considered itself menaced by Germany as much as Russia, Hungary and Romania would have been embroiled in an arms race and possible military confrontation over old territorial and ethnic disputes, and all Eastern Europe would have planned new military alliances, often against each other.

Keeping Eastern Europe out of Nato would have been a recipe for a replay of European tensions of the 1930s vintage; Russia, which knew this very well, wanted this outcome because it believed that this was the only way it could maintain influence in the heart of the continent. But it was Nato which foiled these dangers, and without firing a shot in anger.

Yet until recently and notwithstanding these achievements, Nato looked destined for a genteel decline. Its military operations in Afghanistan and Libya ended without a convincing victory. Meanwhile, a long-running dispute between the US and Europe over the resources which the Europeans devote to their defences went unaddressed.

In the early 1990s, the US accounted for half of Nato's military capabilities. Today, however, the Americans are responsible for no less than 80 per cent of Nato assets, despite the fact that in terms of population and combined size of their economies, the Europeans are actually bigger than the US.

A club of free riders

IN EFFECT, this is not an alliance but a club of free riders, piggy- backing on the mighty US military. And Europe's recent financial crisis made matters much worse: Defence budgets were slashed throughout the continent because cutting back on the procurement of weapons is quick and politically painless. Some officials at Nato's headquarters joked that the acronym for the organisation actually stood for "No Action, Talk Only". And the future looked so grim that former US defence secretary Robert Gates used his last day in office in 2011 to warn the Europeans that they faced a dismal future of collective military irrelevance.

But Russia has yet again saved the alliance: The Russian annexation of Ukraine's Crimean peninsula, the first European land grab since World War II, has shocked Nato into action. At the summit in Britain this week, the alliance will announce the formation of a new Rapid Reaction Corp which could be deployed to reinforce the territory of East European countries which may come under threat from Russia.

And the Russians will be warned in no uncertain terms that the tactics which they used in Ukraine - the introduction of anonymous fighters who undermine a country from within - will not be tolerated, and will invoke an immediate Nato response, including the activation of the famed US security guarantee.

Nato's "relaunch" is risky. Formal vows aside, there are no indications that the Europeans are prepared to spend more on their defences. Nor is there any indication that US President Barack Obama is about to redirect US military resources to Europe. And defending countries against internal rebellions sponsored by Russia is not what the Nato military is trained for, nor particularly good at doing. If Moscow encourages ethnic Russians in the small and vulnerable Baltic states to undermine the internal stability of those countries, it would be tricky for Nato to decide when to go to the aid of these countries.

Difficult but not impossible, for Russian President Vladimir Putin will be committing a grave error if he dismisses the significance of the decisions which will be taken at the Nato summit this week, since the habit of cooperation that the alliance has created gives the organisation a unique advantage which Russia cannot match.

The alliance's biggest asset is its single command, the Supreme Allied Commander Europe, or Saceur, as he is universally known. Saceur knows where all the alliance's troops are and, once he is given the order to create the new force, this will be done in a matter of weeks. It will be Saceur who will have the powers to deploy the force in Eastern Europe as needed, bypassing the need for an elaborate political decision- making.

As Nato has repeatedly demonstrated throughout its history, once galvanised, it is a formidable force. Its hardware alone outnumbers that of Russia by a factor of four to one; Nato's technological lead over Russia is measured in decades, not years.

Nor should Mr Putin ignore the fact that most Nato members who hoped at the end of the Cold War that the alliance would be able to forge a partnership with Russia now realise that this was a pipe dream.

The conclusion that most Nato nations have reached is one which follows the dictum of Mr Robert Strauss, the last US ambassador to the Soviet Union. He prophetically warned his colleagues that, when one dances with a bear, "you don't quit when you're tired; you quit when the bear is tired".