Russian manoeuvres keep the West guessing
The concern of Nato's Supreme Allied Commander in Europe that Russian forces are capable of moving swiftly into eastern Ukraine underscores the uncertain course of this crisis, with Nato and Western diplomats unable to predict Russian intentions.
Nato has been watching Russian exercises and deployments since the outset of the crisis. US sources indicate that some 20,000 Russian troops have been exercising on Ukraine's eastern borders.
Nato commander Gen Philip Breedlove believes that these are "of a sufficient size to pose a potential threat" - not just, he says, to Ukraine but potentially to Moldova as well.
Moldova is a small republic on Ukraine's south-western border where a small pro-Russian enclave - Trans-Dniester - has broken away.
There are a little over 1,000 Russian troops there, ostensibly as a peacekeeping force. It is not clear quite what Gen Breedlove meant, but it seems unlikely that he expected Russian forces to advance across the whole of Ukraine to Moldova.
His point seems to be that there are now sufficient grounds for concern that Russia may seek to exploit this crisis by consolidating its position in other territories that claim support in one way or another for Moscow.
Military men have to look at both capabilities and intentions. The Western military view is that, primed by recent exercises, Russia has gathered sufficient forces at least for an initial thrust into eastern Ukraine.
For now, Russian spokesmen in Moscow insist that this is not their intention. But they said much the same a few weeks ago about Crimea.
From the outset of this drama, it has been especially problematic to determine Russia's game-plan.
At the strategic political level, it seems that the West has greatly underestimated or failed to understand fully the great sense of national humiliation felt by Russian President Vladimir Putin and his circle at the collapse of the Soviet Union and the apparent humbling of Russia.
His frustration at watching more than a decade of Western military involvements overseas was frequently ignored.
Reports of infiltration
The emphasis in the Western narrative focusing on engagement with Russia - however hesitant - and Moscow's recent diplomacy in Syria and Iran - for all its problems - seemed to an extent to confirm those who believed that Russia wanted to play a constructive role.
But President Putin's Crimea adventure has put that all in the balance.
Western diplomats I have spoken to believe that the episode was not explicitly pre-planned, but that the Kremlin moved swiftly to capitalise on a fast-changing situation on the ground - a situation, it must be said, that Moscow sought to influence by stoking up pro-Russian sentiment.
This kind of internal destabilisation is continuing. Western diplomats place some credence in Ukrainian interior ministry reports that hundreds of people have been stopped from crossing into Ukraine from Russia each week.
Some of them are said to be armed, some with equipment like compasses, and some seem to be former - or maybe even current - members of Russian special-forces units.
Such infiltration, along with the springing up of pro-Russian militias, leaves the initiative very much in Mr Putin's hands.
As of today, he may indeed have no desire to move into eastern Ukraine. But what if pro-Russian elements confront Ukrainian security forces?
What if shots are exchanged and lives lost? Will Putin then simply be reacting to the pace of events on the ground? Or will Moscow actually have helped to orchestrate those events in the first place?