From Jonathan Marcus, the BBC:  Nato leaders and planners look at Iran’s continuing nuclear programme – and the steady improvement it has made to the range and capabilities of its missile forces – and are convinced that missile defence is, at the very least, a prudent insurance policy.

The initial defensive screen being unveiled by Nato this weekend rests upon: a network of US early-warning satellites; a new high-powered X-Band radar based in Turkey; and at least one Aegis-equipped US warship, deployed in the Mediterranean, capable of shooting down an incoming ballistic missile.

Some of Nato’s European members will offer elements of their existing air defences – Patriot missiles in Germany and the Netherlands for example – to bolster the system.

Over time Nato’s missile shield will expand with more anti-missile warships. Two land-based missile defence sites are also planned – first in Romania, and later in Poland.

Professor Sean Kay, an expert on the alliance, and Chair of International Studies at the Ohio Wesleyan University, believes that the Obama administration’s phased approach to missile defence in Europe, which forms the basis of the Nato plan, is both prudent and sensible. But he believes that it also has a much broader political significance as well.

"Missile defence," he told me, "is a very important step towards re-invigorating the core collective defence foundation of Nato, which all the allies should appreciate. . . ."

Moscow has signalled its fundamental opposition to the scheme; with Russian generals even going so far as to threaten to deploy nuclear-capable Iskander missiles against Nato missile defence sites in Romania and Poland.

So to what extent does Russia have a point?

Dmitri Trenin, Director of the Carnegie Center in Moscow is one of Russia’s most acute strategic observers.

"Russia," he says, "sees US ballistic missile defence plans as global in scope".

The concern, he believes, is that "strategic defence impacts upon strategic offence; devaluing the deterrent value of Russia’s own nuclear arsenal."

Mr Trenin accepts that the potential impact of the future system in Europe may initially be insignificant, but says that: "Moscow wants both formal assurances and an insight into the system’s parameters, to be confident that the US has no intention of degrading Russia’s own deterrent power, and that the Nato system has no capability against Russian strategic missiles. Washington’s reluctance to give either raises Moscow’s suspicions. . . ."

Dmitri Trenin shares this concern. He says Russian threats are aimed "at waking the European publics to the dangers inherent in Nato’s missile defence plans if no agreement with Russia is reached".

He believes that for all the bluster Russia will act cautiously. But he insists that "a failure to engage Russia on missile defence will be a grave strategic blunder for Washington and its Nato allies. We have a couple of years, I think," he says, "to sort things out."  (graphics: BBC)