The first shots of war between Russia and Ukraine have already been fired - ever since the 2014 Crimea invasion.
Moscow’s forces already occupy parts of Ukraine, in Crimea and Donbas, where there are regular exchanges of fire, along a 250 mile frontline of trenches.
As many as 14,000 have died there in years of fighting, which has settled into trench warfare and could break out at any moment into a much bigger conflict.
Russian President Vladimir Putin has boxed himself in by insisting NATO has to guarantee Ukraine and other border countries will not be allowed to join the organisation.
Under his leadership the Kremlin, according to Ukraine intelligence analysis, desperately wants to create a USSR Mark II.
The Biden administration may be considering a concession, allowing Putin to claim a moral victory back home and saving face.
But the only option would be to tell Putin Ukraine will not be a member of NATO for the time being, leaving the possibility open.
The US has already said it won’t do that and Putin has overplayed his hand by insisting this is his red line, with no give.
So Putin has to do something otherwise he will appear weak back home.
As a wily operator he needs to maximise his gains whilst minimising the consequences.
This means minimum cost and as few Russian troops killed as possible - as that would be a terrible optic back home.
The assessment of many military observers is that it is now “highly likely” he will order Russian troops to attack.
His best option is a “false flag” operation using Spetsnaz- GRU forces to attack pro-Russian separatists in Donbas.
They would be pretending to be Ukrainian forces and would spark a retaliation against so-called Ukraine by regular Russian troops.
This seems transparently crass and treacherous but it’s been done many times before.
Regular Russian forces are already bolstering pro-Russian separatists there - although Moscow maintains that they are not.
If the Kremlin can persuade Russia this is the truth, that Ukraine has attacked, that might play well back home, even though the rest of the world would know it is a lie.
America has already kind of seen off this option by loudly calling Moscow out, saying it has intelligence this is exactly what his special forces are planning.
However this may not be enough to prevent such an operation, that has minimum consequences, could result in a partial land grab, a push further into Ukraine and be completed quickly.
The false flag option is a tough one to resist for Moscow as it ticks all the boxes - despite being pre-empted by the States.
A full-on invasion would be bloody, costly and unlikely as Ukraine’s forces are becoming well-equipped to fight a guerilla war, retreating and flanking Russian troops.
Many Russian troops returning home in body bags would play badly at home and the war could go on for a long time, with no achievable result at the end of it.
This could come in many forms, from Crimea or Belarus on land or both as a pincer movement - also involving amphibious invasion from the sea.
The worrying fact is that 126,000 Russian troops, including ground forces and naval warships, are already in place along the frontline, in Crimea and Belarus and at sea.
And a further 50,000 could reach the frontline, fully-armed and ready for battle in little more than a week of the first invasion shots being fired.
Worryingly there are no guarantees a small incursion would not naturally spread into a much larger, grindingly horrific and bloody one.
There will almost certainly be an escalation in the coming weeks and whilst it is unlikely, all out war remains a possibility.
