It was meant to be a swift so-called special military operation — Russian troops were to roll into Kyiv and Ukraine's government were to capitulate.
But the invasion didn't quite go to plan.
Instead, Russian forces were faced with several setbacks, including logistical issues, equipment issues, morale issues, as well as being faced with a far more formidable opponent in the Ukrainian forces than anticipated.
Now, as Russia's fronts are collapsing and Ukrainian forces take back areas occupied by Russia, Moscow is desperate to save face.
So the question is: what is president Vladimir Putin going to do?
Nuclear: Will he or won't he?
As Ukraine made breakthroughs on the battlefield, Mr Putin raised the stakes of the war by annexing Ukrainian land, ordering a partial military mobilisation and threatening nuclear retaliation.
In a televised address, he said Russia would use all tools at its disposal to defend its "territorial integrity".
"This is not a bluff. And those who try to blackmail us with nuclear weapons should know that the weathervane can turn and point towards them," he said.
Many experts have tended to agree it's unlikely Mr Putin will resort to using nuclear weapons.
But all agree the threat cannot be ruled out and must be taken seriously.
Loading...Jessica Genauer, international relations lecturer at Flinders University, said the chance of nuclear weapons being used has "increased significantly in recent weeks" as more Russians voice their objections to how the war is being fought.
"[Putin] is now facing a precarious domestic situation, with no expectation of upcoming military successes for Russia on the battlefield," Dr Genauer said.
"When we take this into account, combined with the recent successful strike on the Crimea bridge, I do believe that unfortunately the chances of Putin using a nuclear weapon have increased.
"Putin understands that he must do something to show evidence of the Russian power that he has been promising to his people, and has been so drastically underdelivered in this war."
US President Joe Biden said Mr Putin's nuclear threat was the biggest since the Cuban missile crisis, and that Washington was "trying to figure out" Mr Putin's off ramp from the war.
The White House later clarified the US had no indication that Russia was preparing to imminently use nuclear weapons.
Stephan Fruehling, a nuclear weapons and NATO expert from the Australian National University, said throughout history nuclear powers looked for an alternative to using nuclear weapons, but the crisis in Ukraine was different.
"If you look at, for example, the Cuban Missile Crisis … it's the statesman who ended up being very conscious of the risk and very careful," Professor Fruehling said.
"[But] there is a real risk that Putin might look to nuclear use to actually stave off the crisis that he's facing in Ukraine.
"Their fronts are collapsing in Luhansk and in Kherson, so they don't have the conventional power to stop the Ukrainians.
"So the question is … are the risks of further pushbacks in Ukraine bigger than the possible risks of nuclear escalation?"
Putin needs a way out
As a military victory for Russia becomes increasingly unlikely, Mr Putin has chosen to double down.
"However, Putin also understands that Russia does not have military or economic capacity to keep waging this war in the long term," Dr Genauer said.
Professor Fruehling said Mr Putin could use nuclear weapons to end the conflict in one of two ways — either to stop the momentum of the Ukrainian forces, or trigger a reaction by the US, UK and France in order to justify a withdrawal that saves face at home.
"You could imagine a situation where [Putin] uses a nuclear weapon in order to force NATO into the conflict," Professor Fruehling said.
Russian propaganda has justified the recent setbacks on the battlefield by framing the conflict as a war with NATO, rather than Ukraine: if western powers openly entered the conflict, it could suit Mr Putin's narrative.
"But all of this is speculation and nobody really knows," Professor Fruehling said.
"It's been clear that the Russians don't have enough forces for months, but [Putin] has waited until it was too late and he's only declared a partial mobilisation.
"Even if you see nuclear use, you're going to see very small steps into nuclear use."
What would 'small steps' look like?
Russia has one of the biggest nuclear arsenals in the world, and is estimated to have thousands of nuclear warheads in its stockpile, assigned for both long-range strategic launchers and shorter-range tactical nuclear forces.
Tactical nuclear forces are smaller warheads that are used on the battlefield.
But they're not easy to use, especially against Ukrainians operating in small, widely dispersed forces.
"You'd actually have to blanket vast areas of Ukraine with nuclear weapons if you really wanted to stop the Ukrainian army," Professor Fruehling said.
"I think there is a possibility that [Putin] might drop a nuke on Kyiv in order to kill the Ukrainian government, because his original goal was 'denazification'.
"If he manages to kill Zelenskyy and the Ukrainian government with a nuclear weapon, that may be a way for him to declare victory and then possibly withdraw."
How would the West respond?
Last week, former CIA director and retired army general David Petraeus said the US and its allies would destroy Russia's troops and equipment in Ukraine and sink its Black Sea fleet if Mr Putin used nuclear weapons in the conflict.
"To give you a hypothetical, we would respond by leading a NATO — a collective effort — that would take out every Russian conventional force that we could see and identify in the Ukraine and also in Crimea and every ship in the Black Sea," Mr Petraeus told ABC News US.
"It's not nuclear for nuclear — you don't want to get into a nuclear escalation here, but you have to show that this can't be accepted in any way."
Professor Fruehling said the idea that Russia could use a nuclear weapon against a non-nuclear country — given that Ukraine gave up its nuclear weapons in exchange for security guarantees from the US, Britain, France and Russia — would set a "horrible precedent".
"Obviously, we're not interested in normalising the use of nuclear weapons," he said, "but what are you going to do?"
He doesn't believe there would be a nuclear response from the West, or attacks on Russia itself.
"There are some signals that the Americans have threatened Russia that if they use a nuclear weapon, the US would enter the conflict directly and attack Russian forces in Ukraine and I think that that's the most likely scenario," Professor Fruehling said.
"I don't think that you'd see NATO getting involved because it's not an attack on NATO.
"I think the most likely thing would be that Britain, France and the US would enter the conflict in Ukraine and only with air power."
Why might Putin risk nuclear war over Ukraine?
A nuclear attack would completely destroy its target area and trigger a toxic radioactive fallout.
If the destruction of cities like Mariupol is anything to go by, Russia doesn't seem intent on preserving the areas it's fighting to take control of.
It raises the question, what exactly is Mr Putin fighting for?
Dr Genauer said the invasion was being driven by Mr Putin's desire to appear strong and powerful in Russia.
"We have seen Russia use tactics of decimating cities and civilian infrastructure in past conflicts in Chechnya and as part of Russia's military support for President Assad in Syria," Dr Genauer said.
"In both of these cases, whilst of course there are significant differences to the war in Ukraine, this tactic ultimately resulted in breaking the local populations' resistance."
Dr Genauer said Mr Putin would consider it preferable to destroy cities and towns in Ukraine, in an attempt to achieve control, than admit defeat.
Professor Fruehling said the war, ultimately, was about Russia's identity as a colonial empire.
"In many ways, what you see at the moment is Russia reasserting itself, and its own view … as a colonial empire."
Professor Fruehling pointed to the mobilisation happening in Buryatia, Chechnya and Dagestan, where "villages are being emptied" as a sign of Russia's colonialism.
The mobilisation is not being felt in Moscow or St Petersburg.
"It goes to the view of Russia as a colonial empire and the fact that Ukraine for many centuries was governed by Russia," Professor Fruehling said.
"And if you challenge the view that Ukraine is part of Russia, in many ways you actually challenge the view of Russia's existence at the same time.
"Because if Ukraine isn't part of Russia, well, I mean, why should the Buriyats and the Tatas and the Chechens and Dagestan and so on be part of Russia?
"You're not going to see, I think, Russia move away and give up its claims to Ukraine because that would ultimately de-legitimise the existence of modern Russia."