Washington has warned the assault will open with massive cyberattacks and wave after wave of missiles and cannon shells. Combat aircraft will likely cross the border shortly after, with tanks and armoured vehicles hot on their tails.
US news service Politico reports unnamed government sources saying the intelligence detail is "specific and alarming". '
And that's why National Security Advisor Jake Sullivan has issued a curt warning to all US citizens to get out of Ukraine by Wednesday.
While Washington's European allies aren't so convinced, NATO is stepping up preparations to reinforce its eastern flank. Combat aircraft are reinforcing the Baltics. Troops are moving into Poland. Romania and Lithuania are clamouring for help.
The official Kremlin position is that all 140,000 troops will stand down after their "war games" end February 20. It insists the 40,000 troops inside the former Soviet state of Belarus – to Ukraine's north – will return to Russia.
Most of NATO's former Eastern Bloc members doubt this.
They point to Moscow's history of occupying territories – such as their own – under false pretences and never leaving.
And if this was to happen in Belarus, which borders Poland and Lithuania, the security ramifications could be far-reaching.
Save the date
"We don't know what's going to happen, but the risk is now high enough – and the threat is now immediate enough – that this is what prudence demands," Sullivan explained from the White House podium after urging all US citizens to get out of Ukraine, fast.
He, and other US officials, have refused to confirm reports that President Biden had pinned the invasion date to February 17. But the sense of urgency was reinforced.
"We cannot perfectly predict the day, but we have now been saying for some time that we are in the window, and an invasion could begin by Russia in Ukraine any day now," he said.
Pentagon spokesman John Kirby also declined to put a date on the attack. Still, he again emphasised its imminence: "These assessments are coming from a variety of sources. And not, not exclusively just inside intelligence, but also what we're seeing in plain sight."
At the weekend, President Biden repeated that he would do everything to support Ukraine short of war. He reiterated NATO would fight to defend its member states.
"We will defend every inch of NATO territory, every inch of Article Five territory and Russia we think fully understands that message," Sullivan later told US media.
He added the capital city of Kyiv was at risk.
"There are very real possibilities that (the invasion) will involve the seizure of a significant amount of territory in Ukraine and the seizure of major cities, including the capital city."
Such a move would be diplomatically risky for President Putin.
It's not about the threat of Western sanctions. These have long been part of his risk-reward equation.
Putin had promised China's Chairman Xi Jinping that there would be no attack during the Winter Olympics.
The Beijing Games end on February 20.
Under the anticipated timeline, Putin's troops will move four to five days earlier.
Whether or not this angers Chairman Xi could determine the war's success. Putin is relying on China as a means of bypassing any resulting sanctions.
Decades of good undone
Europe is preparing for a resumption of the Cold War. With a difference.
Several former Soviet Bloc nations are now on NATO's side. And these constitute the frontline of conflict with a post-communist, authoritarian Russia.
And President Putin believes their territory is rightfully his.
That's why fear is already spreading through Eastern Europe and the Baltic.
One of his demands is that NATO reset its forces – and membership – back to 1997 levels.
That's when the NATO-Russia Founding Act declared they "do not consider each other as adversaries".
Things have changed.
"If Russia really wants less NATO close to the borders, they'll get the opposite," NATO general secretary Jens Stoltenberg said last week. "We saw that after 2014 when Russia went in and annexed Crimea and went into Donbas."
The Foundation Act began to unravel in August 2008 when the Kremlin sent its troops to occupy parts of neighbouring Georgia. They're still there.
Then, in 2014, it invaded Ukraine and seized the Crimean Peninsula. Unbadged Russian frontline troops and equipment have been supporting separatists in eastern Ukraine ever since.
One of these units was responsible for shooting down Malaysian Airlines Flight 17.
Eastern Europe fears another such "accident".
All it took to trigger World War I was an assassin's bullet – in Austria-Hungary.
For World War II, years of appeasement ended when Nazi Germany manufactured a pretence to invade Poland.
In December, a Ukrainian minister said that a total invasion of Ukraine would spread conflict into Europe and could trigger World War III.
Of trip-wires and troop-corridors
Since 2014, NATO has reversed its retreat from the east. It's one again been reinforcing its defences.
Warships began patrolling the Black and Baltic Seas. Multinational battle groups are in Poland, Latvia, Lithuania and Estonia. Another such unit is in Romania. And most of these nations host visits from NATO "air policing" combat units.
It takes two sides to form a friendship.
"A full-scale Russian invasion would do nothing but galvanise solidarity in the alliance," says former NATO supreme allied commander for Europe James Stavridis. "It would also open the door for permanent deployments into NATO member states on the Russian border, most notably Poland."
British, German, Canadian, and US forces are already in Eastern Europe to deter Putin's ambitions.
These forces are tiny. In total, they barely number 4500 troops.
They would be rapidly overwhelmed by any concerted assault. But their lives will act as a "trip wire" to trigger a much broader conflict.
Now Romania and Lithuania are pleading for a more prominent and effective NATO presence.
It's not an unexpected reaction.
"If the Russians succeed in re-establishing a sphere of influence or of dominating Ukraine, they won't stop there. They will continue," former US ambassador to Ukraine William Taylor told the Washington Post. "The Poles and the Romanians, the Czechs will be very concerned as they see Russian tanks coming west from Russia into Ukraine toward them, and they will ask for reinforcements from the United States."
NATO is already considering sending reinforcement battle groups to Romania, Bulgaria, Slovakia and Hungary, if the host countries ask for them.
On Saturday, the US surged 3000 additional soldiers to Poland – on top of the 3000 already sent there. These are said to be taking up positions to assist any refugees from Ukraine.
Another 1000 US soldiers will arrive in Romania to reinforce the 900-strong contingent stationed there.
A flight of US F-15 interceptor fighters also landed in Poland at the weekend. Notably, they were carrying live AIM-120 medium-range air-to-air missiles. These counter a move by Russia to send MiG-31 Foxhound aircraft armed with hypersonic missiles to its Kaliningrad enclave between Poland and Lithuania.
Four nuclear-capable USAF B-52 bombers have arrived in the United Kingdom. And surveillance aircraft have been swarming around Ukraine in increasing numbers over recent weeks.
Meanwhile, four US destroyers have been sent into the Mediterranean to reinforce the aircraft carrier USS Harry S. Truman. It already has four destroyers, a submarine and a cruiser in company.
