When is an invasion not an invasion? No Russian tanks have rolled across Ukraine's eastern frontier and no spearhead infantry units are ravaging the post-industrial wastes of the Donbas, yet the Kremlin has still managed to seize de facto control over a string of towns inside its neighbour.
This shadowy operation in eastern Ukraine reveals much about the temperament of President Vladimir Putin. Ordering a conventional military invasion - a sort of Operation Barbarossa in reverse - would have been far too obvious for this KGB graduate. While NATO's high command fretted over the presence of between 35,000 and 40,000 Russian troops on Ukraine's border, he chose a more subtle method of coercing his neighbour.
Subversion from within - not attack from without - has emerged as Mr Putin's favoured technique for controlling events in Ukraine. A full-scale invasion may yet happen, but for now at least, the Kremlin has chosen to wage a deniable war in the twilight.
Stage one of this operation began 10 days ago when armed men seized government buildings in Kharkiv, Luhansk and Donetsk, the capitals of Ukraine's three eastern regions with large Russian minorities. Then, on Saturday and Sunday, this campaign suddenly escalated. Within the space of 24 hours, the gunmen occupied police stations and other official buildings in six towns across the Donetsk region.
Their modus operandi summoned comparisons with the fate of Crimea, which fell under de facto Russian control in a few days in February. As in Crimea, the men in the vanguard of these assaults flew Russian flags from their conquests. As for their aim, they demanded total autonomy for their home regions - or straightforward union with Russia.
William Hague, the Foreign Secretary, drew an explicit parallel with Crimea on yesterday (Monday), noting how the "forces" in action in eastern Ukraine were "behaving in exactly the same way as what turned out to be the Russia forces in Crimea". All this amounted to a "gross, deliberate and premeditated violation of the independence and sovereignty of Ukraine", Mr Hague added.
So will eastern Ukraine go the way of Crimea and end up as another region of Russia, or is Mr Putin playing a more subtle game? No one can doubt that he has already achieved his first objective, namely to impale Ukraine's post-revolutionary government on the horns of an impossible dilemma. If the country's new leaders rise to this challenge and deploy their army to wrest back the occupied towns and government buildings, then Russians might well be killed - and Mr Putin would have his pretext for a full-scale invasion.
Hour after hour, Russian television pumps out propaganda about how Ukraine has fallen into civil war and the February Revolution brought "fascists" and "extremists" to power. Mr Putin would claim any bloody incident as proof of his case. If he chose to invade, he would present this onslaught as the only way of calming the chaos, hoping that we will forget his part in causing it.
If, on the other hand, Ukraine's leaders ignore the provocation and leave towns like Slaviansk in the hands of their enemies, then the central government would steadily lose control over the east. Groups of masked men would carry out one occupation after another. The map might remain the same, but in reality, the border between Russia and Ukraine would be submerged as the Kremlin proceeded with the creeping de facto annexation of large areas of its neighbour, building by building and town by town.
In theory, the government in Kiev has made its choice. On Sunday, Oleksandr Turchynov, the acting president, gave the armed men until yesterday to leave all public buildings or face an "anti-terrorist operation". But the deadline attached to this threat expired with no sign of an assault, suggesting that Mr Turchynov and his colleagues are still wrestling with their dilemma.
That will not have disappointed Mr Putin. He has carefully left his adversaries with a way out: they can escape from their agonising predicament, but only by doing exactly what he tells them.
Russia has already spelt out its objectives in Ukraine: the Kremlin wants the country to have a new constitution allowing total autonomy for the eastern regions. In addition, Russia demands that this new settlement must guarantee Ukraine's non-aligned status - in other words, rule out membership of NATO or the European Union for ever. Taken together, these diktats would guarantee that Ukraine remains firmly inside Moscow's orbit.
With the manoeuvres of the past few days, Mr Putin hopes that he has won the leverage to bend Ukraine's leaders to his will. "There are two possible interpretations of these events," says Dr Alex Pravda, a Russia specialist at St Antony's College, Oxford. "Either this is part of a long-term strategy to partition Ukraine. Or it's a series of tactical moves designed to leverage influence over Kiev. I think it's the latter."
But Mr Putin is running risks. The first is that Ukraine's leaders might choose the first horn of the dilemma and meet force with force. If they wrest back a town, killing a dozen or so Russians in the process, then Mr Putin could find himself under pressure to invade, whether that is part of his plan or not. "More than five or 10 casualties would be a very difficult situation for him to manage," says Dr Pravda. "He's trying to create a chaotic situation where he has managed leverage, but it's high risk."
Moreover, the people of eastern Ukraine might not behave in quite the way that Mr Putin hopes. If he hoped to trigger a groundswell of popular revulsion against the revolutionaries in Kiev, then this has not happened yet. Only a few hundred people have been directly involved in the occupations, supported by relatively modest crowds of demonstrators.
Unlike in Crimea, ethnic Russians are a minority in eastern Ukraine, albeit a sizeable community of 38 per cent in the Donetsk region. Nonetheless, opinion polls suggest that support for joining Russia is a minority cause. Over 83 per cent of voters in Donetsk backed independence for Ukraine in a referendum accompanying the dissolution of the Soviet Union in December 1991. So it is far from obvious that Mr Putin is working with the grain of popular opinion. If he does invade, the inhabitants of eastern Ukraine could wage an eternal partisan war behind the lines even if he achieved a conventional military victory.
There is, however, a still more important question. What exactly are Mr Putin's long-term goals? In his speech announcing the return of Crimea to Russia last month, he denounced the iniquities of the post-Cold War settlement, forced upon his country at its moment of maximum weakness. He promised a rapturous audience that he would reverse those injustices and restore Russian greatness.
That ambition clearly included annexing Crimea. But does it also require a forcible redrawing of the map of eastern Ukraine to incorporate more Russian-speaking regions into the mother country? And what about the Baltic states, all of them members of NATO and the EU, while also being home to significant Russian minorities?
If Mr Putin wants to change the world bequeathed by the West's victory in the Cold War, then where exactly does this mission end?
Perhaps he is only really interested in the Slavic heartland of the old Soviet Union, namely Russia, Ukraine and Belarus. Equally possible is that he does not know the answer to that question. But the fate of eastern Ukraine in the weeks ahead will help to reveal how far Mr Putin is prepared to go in his burning ambition to restore Russia's greatness.