Ukrainian government troops have made significant gains in recent days, pushing pro-Russian rebels out of a string of towns in the east. The rebels have retreated to Donetsk from Sloviansk, for weeks a powerful symbol of their resistance to Kiev.
So are Kiev's forces winning the conflict? Alexander Golts, a military expert and deputy editor of the Russian online newspaper Yezhednevny Zhurnal, examines the new stand-off.
Ukrainian politicians say a fundamental turning point has been reached in the conflict. But the experience of similar conflicts elsewhere - with a regular army confronting paramilitary units - provides no basis for such claims.
Nobody has succeeded in defeating paramilitaries who are embedded in a city, virtually turning its residents into a human shield - the Americans did not win such a conflict in Mogadishu, Somalia, nor did the Russians win in the Chechen capital Grozny.
In such a situation a regular army cannot use its superiority in heavy weapons over rebels - weapons such as armoured vehicles, aircraft and artillery.
The army may manage - after huge efforts - to capture one town, destroying it with heavy artillery, only to find that the rebels have simply moved to another town. That town in turn has to be taken by storm, and then the same thing happens in a third town.
It appears that the same thing has happened now in the Ukraine conflict.
It took the Ukrainian regular army several weeks to surround Sloviansk. The Ukrainian forces lost several planes and helicopters in that operation.
The rebels led by Igor Strelkov - a military adventurer from Russia whose real name is Igor Girkin - moved to Donetsk and Luhansk to escape the siege.
Now the Ukrainian army is doomed to suffer losses in a siege of regional centres, where each district can be turned into a centre of armed resistance.
It would be wrong to state that the Ukrainian army has become more efficient than when the conflict started.
It is simply that, as always in such wars, more decisive commanders have taken charge, men who do not hesitate to use heavy armour, artillery and aircraft.
In fact Ukrainian National Guard volunteer units are playing a significant role. They are ideologically motivated, better paid than the army, and evidently making the armed forces more effective.
The pro-Russian separatists cannot continue the fight without support from outside.
Their ammunition is running out, they constantly need new weapons. And they need an inflow of so-called "volunteers" - and we know where they come from.
The fighters also need training - and somewhere to train.
So in theory sealing the border with Russia could end the conflict - but in reality Ukraine does not have sufficient forces to do that.
Whether or not Donetsk can be taken without large-scale damage depends directly on the strength of the defending rebels.
We do not know exactly what numbers and equipment the rebels have managed to concentrate in Donetsk. But fighting in a modern city is always an army's nightmare.
In 2003 the Americans were so daunted by the task of assaulting Baghdad that they studied how the Russians stormed Grozny - and that attack on the Chechen rebels was certainly no great success.
The Ukrainian army will probably try to use its numerical superiority - experts reckon that 30,000 regular troops are facing a maximum of 10,000 separatists.
The logical tactic in conducting such a siege of Donetsk and Luhansk would be to put the rebels under pressure simultaneously in several places, forcing them to dissipate their energies.
Igor Strelkov's response might well be to create mobile groups of 200 to 300 fighters, equipped with mobile rocket launchers and anti-tank weapons.
The rebels will try to cut communications links to Donetsk, to block the deployment of some 6,000 Ukrainian troops freed up by the seizure of Sloviansk.
Three bridges have been blown up on roads leading to Donetsk, and Ukrainian troops will doubtless run the risk of rebel ambushes on major roads.
So far there is no clear answer to the question: which side will be first to incur the local residents' hostility? The rebels, whose appearance will be a signal of impending clashes? Or the regular troops, whose use of heavy weapons will cause civilian casualties and destroy homes? Either way, there is no early end in sight.
The Kremlin's position is of course crucial in this situation.
Under the threat of more serious Western sanctions it appears that President Vladimir Putin has rejected the idea of direct military intervention disguised as a peacekeeping operation.
Most likely the secret support for the rebels, through supplies of volunteers and arms, will continue.
That support does not go far enough for those fighters who want to attach south-eastern Ukraine to Russia - a part of Ukraine already described by Kremlin propagandists as "Novorossiya" (New Russia).
The Kremlin will try to keep Mr Strelkov in Ukraine with his followers, armed with Kalashnikovs. Otherwise they would stir up trouble for Russia, armed and angry.
So for now Moscow's goal will be to maintain controllable chaos in Ukraine. That policy will also serve to show the Russian people that any attempt at a Ukraine-style "colour revolution", any attempt to get rid of the authoritarian state, will result in chaos and civil war.