The moment when Vladimir Putin decides whether or not to order an offensive against Ukraine is imminent, Western officials believe.
They regard an unprecedented build-up of Russian forces around their neighbour’s borders as a moment of great jeopardy, it is understood.
If a large-scale invasion were to occur it would likely be fast and deadly with missiles launched from the sea against Kyiv and ground troops cutting off Ukraine‘s most capable armed forces based in the east of the country, according to Western assessments.
It means the crisis is approaching the moment when Western officials will be on their highest level of alert for what could be a bloody and destructive barrage.
“It would be a massacre,” a Ukrainian official said.
Western officials stress that an invasion is still not inevitable.
However, they point to the massing of some 130,000 Russian troops around Ukraine’s borders and off the coast – the biggest mobilisation of combat forces in Europe since the end of the Cold War.
The units and equipment that have been assembled – and more are still in transit – are designed for an invasion, not to conduct exercises, as Mr Putin has claimed, according to Western assessments.
In terms of scale, the build-up includes about 60% of Russia’s ground combat power and around half of its air power.
There are also warships in the Black Sea capable of launching an amphibious landing on Ukraine’s coast and firing precision missiles against targets across the country.
A full-scale attack would probably happen on multiple fronts, including a rapid assault on Kyiv to neutralise the country’s political and military decision-making capabilities.
A key factor in being able to seize the capital would be to encircle Ukrainian troops in the east first to prevent them from hampering the mission.
Any action could – as has been talked about repeatedly by British and American ministers – begin with some kind of false-flag provocation in the part of eastern Ukraine under control of Russia-backed separatists.
Despite the grim assessments of the situation, there does still remain hope for some kind of diplomatic breakthrough but the signs are not good.