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A week of high-stakes diplomacy lies ahead that could end in invasion and war - or compromise and an easing of tensions.
Skilful creative diplomacy will be needed as Russians meet Americans in Geneva, then NATO on Wednesday and end the week face-to-face with Europeans.
The omens are not great. Russia is already threatening to walk out before the talks have even begun. Their chief negotiator says they may not last more than a day.
Some of that may be posturing but it shows the challenges ahead. Each side has conflicting demands.
Moscow says it wants a bar on further NATO expansion, and for the alliance to stop activities in those central and European countries that joined it after 1997.
Russia's opening position is a nonstarter say the Americans.
America hopes it can deter Russian aggression against Ukraine with a mixture of carrot and stick.
The carrot is the willingness to discuss reducing military exercises in the east and missile deployments in Ukraine.
Americans hope a compromise on that could give the Russians a way of climbing down without losing face.
The American stick is the threat of punitive sanctions should Russia attack Ukraine. These would hit Russia financially and economically.
But the two sides are arguing over two different things. For the West, it is about principles.
The principle is that any sovereign country has the right to join NATO should it want to and that any sovereign country should not have its borders altered by force by another.
For Russia, it is more psychological. A country that has been invaded from the West frequently through history is paranoid about its borders. It sees NATO spreading inexorably eastwards.
Ukraine is particularly sensitive for Russians, for so long under their sphere of influence but now leaning westwards.
Vladimir Putin has said Ukrainians and Russians are the same people.
The talks would not be happening if Russia had not parked tens of thousands of troops on Ukraine's borders, even if Moscow insists they are only on exercises.
America cannot afford to look like it is rewarding such brazen behaviour with concessions.
But equally, it cannot afford the humiliation of Russia invading a friend and ally. The damage to the West's power and prestige would be considerable.
For Joe Biden, already floundering in the polls and seen as weak by many, the political impact would be substantial.
Putin knows that but also knows an invasion will be popular only if it is swift and overwhelmingly successful.
That is a big risk. Ukrainians are outnumbered by the Russians but would be fighting to save their homeland and are better equipped than they were when Russia took Crimea in 2014.
Putin also knows the Russian economy can ill afford the kind of sanctions the West is threatening.
It is in the interests of both sides to find a way out but Ukraine is a fault line in East-West relations and the slightest misstep could lead quickly to escalation and conflict.
Putin knows sanctions would be painful but might be tempted to seize land with a blitzkrieg advance, hoping to trade it for concessions later.
He may even use these talks as a pretext to do so. There are many possibilities intriguing observers as diplomats prepare to sit down and lock horns.
If they fail, the outcome could be grave for the region and beyond.