Analysis: Can Nato face up to Russia task?
"Let's be clear from the outset. The Cold War is not back. Russia for all its military power and posturing is not the Soviet Union of old. This is not going to be a re-run of the ideological battle that divided the world for most of the last century."
That in a nutshell was the view of one senior Nato diplomat who I spoke to recently.
But something has changed in the wake of Russia's seizure of the Crimea and its continuing military threat to eastern Ukraine.
Moscow has broken with a pattern of behaviour that has characterised diplomacy in Europe since the end of the Cold War and arguably one that has held sway in western Europe since the collapse of Nazi Germany.
The idea that disputes will be settled by diplomacy rather than force; that the currency of power is increasingly economic might rather than military.
Worse still, Mr Putin's Kremlin speech some 10 days ago signalled that this might not end here. Russian spokesmen may say they have no desire to move troops into Ukraine but their deployments signal otherwise - and that is precisely what they are intended to do.
How far will Putin go?
Mr Putin's speech was significant not least because he looked to mean what he said. The French have a word for it - "revanchisme" - a term incorporating the concept of revenge or restitution after a humbling defeat; something France experienced at the hands of Prussia in 1870, and something that many Russians - certainly those in Mr Putin's circle - believe they suffered at the hands of the West with the collapse of the Soviet Union.
Mr Putin signalled that Russia was back and that it intended to impose itself in its own back-yard and for many in Russia, Ukraine is the biggest back-yard of all.
Quite how far Mr Putin is prepared to go is unclear. But the threat is evident - to Ukraine; to Moldova (where there is the possibility of Russia seeking to formalise the take-over of the break-away enclave of Trans-Dniester). And if to Moldova where else ?
That is why countries like the three Baltic Republics are so relieved that they joined Nato 10 years ago. And that is why all of a sudden the protection and reassurance of its own members is so high on Nato's agenda.
The Atlantic Alliance cannot know the Kremlin's intentions. It can only look at capabilities and extrapolate from the signals and actions so far.
So what can Nato do? Its first step is to reassure its uneasy allies in northern Europe.
Support for Ukraine
In the first instance there could be a whole host of small military deployments to beef up the presence of Alliance units in the Baltic Republics and in Poland. Small-scale exercises could give an almost permanent Nato military presence in these countries for the immediate future if that is what Alliance leaders decide.
There is also going to be more support for Ukraine. Not to bring it into Nato's fold - the current Ukrainian government seems to have no desire to join the Alliance.
But Ukraine is a Nato partner. It has already had help with improving civil control over its military; with defence planning and so on. Expect more of this kind of help with potentially also non-lethal assistance to help make its largely moribund military more effective.
Nato is also signalling that its direction of travel will not change. Some critics have suggested that Nato expansion since the ending of the Cold War has in some sense prompted Russia to act in Ukraine; that it has provoked a sense of encirclement in Moscow.
Nato diplomats reject this out of hand. Secretary General Anders Fogh Rasmussen, in an article published on Tuesday in newspapers in all 12 of the countries who have joined Nato since the end of the Cold War, insists that enlargement has been good for Europe, for Nato, and for the new members themselves. He insists this process will continue and it is up to individual countries to determine their own alliances.
By coincidence this week's Nato meeting will mark a triple anniversary: 15 years since Poland, Hungary and the Czech Republic joined the Alliance; 10 years since the accession of Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia joined; and five years since Albania and Croatia acceded. It's an anniversary that Nato ministers will mark with a brief ceremony and its symbolism will not be lost on Moscow.
Above all, Nato ministers need to plan for the future.
Just how should the Alliance respond to the new wind blowing from Moscow?
Work will be commissioned this week that will feed in to the next Nato summit to be held in Wales in September.
The rules of the game
This will give direction and guidance to a series of studies. Just what will the future relationship between Nato and Russia be like? Is this a temporary cooling? Or as one Nato diplomat put it to me: Is the whole effort to build a partnership with Moscow in jeopardy?
If the security rules of the game have changed in Europe, what will be the military implications? Cash-strapped European countries may have to look again at defence budgets or, at the very least, take the pooling of capabilities more seriously.
The US will have to underscore its continuing commitment to European security in some tangible ways. There may need to be a different pattern of exercises. The force structure may need to be looked at too.
It simply will not be a case of business as usual, not because the Cold War is back, but because Nato's core purpose - the territorial defence of its own members - has suddenly become rather more important than it was a few weeks ago.