Will Vladimir Putin order direct military intervention in Ukraine? Russia already enables a free flow of Russian volunteers and mercenaries to fight against government forces in eastern Ukraine. It is supplying the rebels with weapons, vehicles and ammunition. It is shelling and rocketing Ukrainian territory daily, and promotes the portrayal of the Kiev government as cruel, illegitimate fascists in Russian-language media. The key leaders of the rebels, like Igor Strelkov, Alexander Borodai, Igor Bezler, Nikolai Kozitsyn and Vladimir Antyufeyev, are Russian citizens or Russian nationalists from ex-Soviet territories under Russian control.

Despite this the rebels have lost ground, and have retreated to their redoubts in the big eastern cities of Donetsk and Lugansk, the town of Gorlovka (Horlivka in Ukrainian) and south-eastern Lugansk region.

In recent days, clues to a possible template for direct Russian military intervention have emerged. Even as Ukrainian forces win territory, Russia and the rebels have tightened their grip on a stretch of the Russia-Ukraine border in south-east Lugansk. Hundreds of Ukrainian border guards and supporting troops were either forced to retreat under fire (the Ukrainian version) or deserted (the Russian version) into Russia. Meanwhile Russia has positioned tens of thousands of troops and aircraft near Ukraine.

In the past Russia has justified – and to some extent designed – its interventions in such a way as to defend them internationally on the grounds of ‘we’re only doing what you did in Iraq/the Falklands/Bosnia/Kosovo/Libya.’ On Monday, the Russian foreign minister, Sergei Lavrov, sought the support of the UN and other international organisations for a ‘humanitarian mission in south-eastern Ukraine'. What he meant by a ‘humanitarian mission’ he did not specify.

But the likely rejection of Russian participation in a ‘humanitarian mission’, on the grounds that Russia needs to halt its covert military mission in the area first, wouldn’t necessarily stop Russia going ahead with a ‘humanitarian’ intervention on its own. And although such an intervention would lead to more bloodshed and horror, Russia’s interference in Ukraine – without which the current war would not have begun – has already caused plenty of damage. More than a thousand people, combatants and civilians, have been killed, tens of thousands have fled, homes have been destroyed, bridges have been blown up, businesses have collapsed, education has been halted, and infrastructure has been ruined. Lugansk is said to be without electricity, water or mobile phone coverage.

Dressing invasion in a ‘western’ costume has a military as well as a moral and legal aspect. The first step might be the imposition of a ‘humanitarian’ no-fly zone over eastern Ukraine, with the Russian air force and surface-to-air missiles preventing Ukrainian aircraft operating over rebel positions. But Russia has provided the rebels so lavishly with missiles – and, Ukraine claims, shot down at least one Ukrainian plane directly – that to some extent there is a no-fly zone already.

A fully fledged ‘humanitarian’ invasion of eastern Ukraine would, if it followed the US pattern, be preceded by a lengthy aerial campaign to destroy the Ukrainian air force and all the country’s air defences, giving Russia complete air superiority. But apart from the fact that the Ukrainians would fight back, this would involve the Russian air force bombing air bases in western Ukraine, provocatively close to the border with the EU and Nato.

It may be that Russia will not up the level of its intervention beyond the support it is giving the rebels already, but is preparing the ground for the separatists to retreat to an area immediately adjacent to Russia – Lugansk city perhaps, or just east of it – which they can hold indefinitely under the cover of Russian supplies, artillery fire and the occasional cross-border raid. Still, it is not easy to see how Putin can extricate himself from the disaster he has made in this way with political safety, or with the continued respect of his ex-KGB peers and rivals, whose esteem, I suspect, matter more to him than any degree of western sanctions.

Donetsk, the rebel-held capital of the east, is the key. If Donetsk falls to Ukrainian government forces, Putin will be seen to have been defeated, regardless of his audacious theft of Crimea and the future fate of Lugansk. The rebel commander in Donetsk, Strelkov, has become a hero to Russians. Perhaps the Russian cavalry will ride to his rescue. Perhaps he will die in battle. Perhaps this avatar of a White Russian general will yield to local figures, civilians, politicians, people who, unlike him, are natives of eastern Ukraine, who would be capable of making a deal with Kiev. But he is in an odd position: both Putin’s standard bearer, and, for that reason, a threat to Putin. He may need to watch his back.