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This is perhaps the least likely of outcomes. Mr Putin has marched his troops up the hill and they are not going to return to their barracks without him registering some kind of victory. President Putin has both international and domestic concerns. Perceived weakness will serve him badly.

For all his talk about Russia and Ukraine's historic destiny, which is dismissed by many western commentators as hyperbole, he has real concerns - not just about what he sees as Ukraine's drift into Nato's orbit, but also about the consequences of a viable democratic system being established in what he regards as part of the Russian heartland.

President Biden is not going to accept Russia's demand for a veto over Ukraine's potential membership of Nato. Though in reality any such membership is a long way off. So could Mr Putin be offered some wider diplomatic benefits to forestall war?

The Russian president has already registered a small diplomatic victory of sorts insofar as this video summit with his US counterpart has happened at all. Russia's ongoing troop build-up around Ukraine has forced the US president to put Moscow's concerns at the top of its foreign policy agenda. It's a potent reminder that for all the talk about Washington's new strategic focus being on China, it cannot ignore its long-standing commitments in Europe and, if Moscow seeks to do so, it can - temporarily at least - re-order the Biden administration's strategic priorities.

Russia is preparing for the military option whether it happens or not. This could take a variety of forms from a large incursion, to a significant invasion of the eastern part of Ukraine. One aim would be to bring the main fighting elements of the Ukrainian army to battle and to inflict such a defeat upon them that the Kyiv government has to rethink its position.

Invading territory amidst a hostile population has significant risks. Ukraine's armed forces have had some western weaponry and training and are much improved since 2015. However, Russian forces have also improved over recent years. The firepower Russia is building up is impressive. For all the talk about Ukrainian sovereignty Nato cannot and will not come to Ukraine's aid.

And additional weapons supplies might simply contribute to Russia's justification for war.

Moscow's calculation of the costs of conflict may also be influenced by previous military deployments. While the West currently sees military engagements through the prism of the strategic defeats in Iraq and Afghanistan, Russia may take a very different view. Its operations in Georgia, its seizure of Crimea, its combat in eastern Ukraine - not to mention its involvement in Syria - may all be seen by President Putin as relative victories.

The US statement released after the Biden-Putin talks spoke of the two leaders tasking their teams to "follow-up", suggesting that at the minimum, the dialogue might continue and channels remain open.

So what are the chances of avoiding war? And what should we look for that will indicate the imminent outbreak of hostilities?

Michael Kofman notes that "war is a matter of politics, and the signs will largely be political". "The Russian official position is a non-starter in many respects and does not offer room for optimism that an agreement may be reached," he says.

On the military side, while an invasion is not imminent, there are a host of indications and warnings that might be present in the weeks and days running up to any operation. The nature and disposition of Russian forces and their activity is one sign.

For now, Michael Kofman's view is that the tensions will continue. "As it stands, Russia is likely to keep building up forces, and if they intend to commit to this course of actions, back them with the support and logistical elements required for a large-scale operation."

Jonathan Marcus is honorary professor at the Strategy and Security Institute, University of Exeter, UK