Russia has amassed more than 100,000 troops, with tanks and other heavy weapons, on its border with Ukraine in recent weeks, sparking fears that Europe is teetering on the brink of its first war in decades.
The threatening display has the West worried that Moscow is trying to overturn one of the outcomes of the Cold War, when Ukraine broke away from its political sphere and became an independent state.
Russian President Vladimir Putin has frequently said that the collapse of the former Soviet Union was the greatest geopolitical catastrophe in the 20th century.
After a series of diplomatic talks last week — between Russia, the US, the EU and the North Atlantic Treaty Organization (NATO) — failed, tensions have further escalated.
The White House has said that Russia could attack Ukraine at "any point", while NATO Secretary General Jens Stoltenberg this week said that "there is a real risk for new armed conflict in Europe".
In Brussels, the mood is also jumpy, with a top EU diplomat telling the BBC that "Europe is now closer to war than it has been since the break-up of former Yugoslavia".
Ukrainian President Volodymyr Zelensky released a video address to the nation on Wednesday, urging Ukrainian citizens not to panic over fears of a possible invasion.
However, he said, the country had been living with the Russian threat for many years and should always be prepared for war.
Poland's Foreign Minister, Zbigniew Rau, said the risk of war in Europe was greater than at any time in the past 30 years.
The United Kingdom has responded by delivering a batch of anti-tank weapons to Ukraine this week, while Sweden and Denmark have both strengthened their presence near the region.
Russia has denied planning a new military offensive, but has made several demands, with the warning that it will take unspecified "military-technical measures" if they are stonewalled.
Moscow said it began its military build-up along the Ukrainian border because it could no longer "tolerate" NATO's eastward expansion and "gradual invasion" of Ukraine.
It has requested that NATO commits to a binding promise never to admit Kyiv to the alliance, and wants a formal assurance from the West to cease military co-operation with Ukraine.
So far, those demands have been met with a firm 'No'.
So how close is the situation to war?
Analysts say full-blown war unlikely
Paul Dibb, emeritus professor of strategic studies at the Australian National University, said the situation had become dangerous, but that he did not believe there was a threat of major military action.
Analysts suggest a more likely scenario is that of a limited Russian invasion along the separatist-controlled territories in eastern Ukraine that it seized in 2014.
Mr Putin might also launch a limited incursion into south-eastern Ukraine to join the country's industrial heartland, Donbas, with Russian-occupied Crimea.
"As we speak, there's evidence that further Russian troops are being brought in from the Russian far-east around Vladivostok into the area just north of Ukraine," Professor Dibb said.
"My view is [that], if Putin intended to have an a military attack, even a partial one, we'd see those troops being built up further in the next few days and weeks."
This week, Russia has been further beefing up its presence near Ukraine with the arrival of troops in Belarus for planned military exercises between the two countries next month.
Belarus is a Russian ally and having troops on its territory would enable Moscow to invade Ukraine from the north.
Cyber attacks over invasion?
Professor Dibb, a former deputy secretary of the Department of Defence, said the most likely initial scenario is that Russia will mount more crippling cyber attacks.
Just last week, Ukraine was struck with a cyber attack that defaced its government websites.
The Ukranian government pinned the blame on Russia.
"My personal view is he may well start, as it did in Crimea, with a cyber attack … And let's acknowledge that the Russians are very good at cyber, one of the best in the world," Professor Dibb said.
Cyber attacks could potentially cut off electricity and energy supplies at the peak of winter, and allow Russia to spread disinformation and propaganda to Ukraine's 8.3 million Russian population into insurgency operations, he said.
Alexey Muraviev, a national security and strategic expert at Curtin University, also says Russia's priority is not the occupation of Ukraine, but instead to have its agreements met to stop NATO's expansion any further to the east.
"For Russia, occupation of Ukraine is not the number one or number two priority … the Russians are more interested in reaching some consensus with Washington and Brussels," he told the ABC.
The West calls for sanctions over strikes
With Ukraine outside NATO and not benefiting from the alliance's security guarantees, the US and its European allies have made it clear that they are unlikely to directly intervene militarily if Russia strikes.
Instead, some have been sending military aid to Ukraine and have raised the prospect of new sanctions on Russia, possibly the severest yet, in the event of an attack.
Germany's Foreign Minister, Annalena Baerbock — whose talks with her Russian counterpart, Sergey Lavrov, also ended with no breakthrough last week — said Moscow would pay the price if it moved on Ukraine but that diplomacy was "the only way".
"Each further aggressive act will have a high price for Russia, economically, strategically, politically," she said.
US President Joe Biden predicts that Russia will "move in" on Ukraine.
He said Moscow would pay dearly for a full-scale invasion, with its businesses possibly losing access to the US dollar, but suggested there could be a lower cost for a "minor incursion".
The Biden administration has prepared a broad set of sanctions and other economic penalties to impose on Russia in the event of an invasion.
However, Mr Biden said NATO allies are not united on how to respond, depending on what exactly Mr Putin does, saying "there are differences" among them and that he was trying to make sure that "everybody's on the same page".
He added that a third summit with Mr Putin was "still a possibility" after the two leaders met twice last year.
China looming in the shadows
Professor Dibb does not think new sanctions will have any impact "whatsoever" in deterring Mr Putin from making moves on his neighbour.
"Russians are hard people. Their history tells us that they used to absorbing hard measures," he said.
Russia has been subject to curbs since its annexation of Crimea, a conflict that has seen more than 14,000 people killed in nearly eight years of fighting between the Russia-backed rebels and Ukrainian forces in Donbas.
Further punitive measures were added after a former Russian spy was poisoned in Britain in 2018 and following an investigation into alleged Russian meddling in the 2016 US presidential election won by Donald Trump.
Professor Dibb warned that Mr Putin knows America and European countries are not willing to confront him with war, and that China will be watching closely.
"China and Russia are increasingly showing signs of being very, very close strategic partners, and not just politically and economically, but militarily," he said.
"If Russia uses military force and gets away with it, then China may be further encouraged to push ahead with its military threats to unify Taiwan."
ABC/wires
