Russia has amassed around 100,000 troops on Ukraine's border, leading to fears that Europe is at its greatest risk of war in decades.
Russia President Vladimir Putin has denied he plans to invade Ukraine, but the US, the UK, NATO and the EU have expressed scepticism.
US intelligence found Russia could invade Ukraine any day now, while the UK alleged the Kremlin was seeking to install a pro-Russian leader in Ukraine — an accusation denied by Moscow and by the politician referred to.
The US State Department is so worried about a Russian invasion that it has ordered the families of all American personnel at the US embassy to leave Ukraine.
On Monday night, the Department of Foreign Affairs and Trade increased its travel warning for Ukraine to "do not travel" and said Australians in the country should "leave now" due to the "increased risk of armed conflict".
But analysts have told the ABC that while Russia is unlikely to try to take over the entirety of Ukraine, it could be considering an invasion of separatist-controlled territories.
So, given that a military escalation between the countries is looking more likely, what are some reasons Russia might invade its neighbour?
To counter the West
Russia maintains that after the Cold War, the US and the North Atlantic Treaty Organization (NATO) promised they would not expand eastward to the former Soviet states.
But Russia became alarmed by NATO's increasing closeness with Ukraine, which is seen by Moscow as a buffer between it and the West.
"For us, it's absolutely mandatory to ensure Ukraine never, ever becomes a member of NATO," Russian Deputy Foreign Minister Sergei Ryabkov said at a media conference earlier this month.
NATO's position is that Russia doesn't get to choose which countries join the alliance.
One of the justifications Russia gave for going to war with Georgia in 2008 was because it believed the former Soviet country was getting too close to the West.
To unite former Soviet countries
Mr Putin has long believed that Russia and Ukraine should be reunited as one country.
Ukraine gained independence following the disintegration of the Soviet Union 30 years ago.
In 2014, Russia annexed Crimea after Ukraine ousted pro-Russian leader Viktor Yanukovych — an ally of Mr Putin's.
Russia argued it had a historical claim to the territory.
"Russia needs to reinvent itself as a modern state and stop clinging to the, let's say, the idea of the reconstruction of the Soviet Union," he said.
"It's already gone."
To expand domestic influence
Mr Putin's popularity increased after the annexation of Crimea, as well as around the time of Russia's brief war with Georgia.
While Mr Putin has not confirmed whether he intends to run for re-election, he signed legislation last year allowing him to remain in office for another two six-year terms — until 2036.
Although Mr Putin has seen his domestic popularity increase after military incursions into former Soviet territories, Alexander M Macroux told the Harvard Gazette there was little public support in Russia for an invasion of Ukraine.
"The Russians have made it clear — it's not like they want Ukraine. That area of eastern Ukraine is an economic disaster," he said.
"And if Ukraine is further split between pro-Russian and pro-Ukrainian, it means that they have less influence there than they did before when they could convince the whole country sometimes to be more pro-East."
To ease crippling sanctions
The US has numerous financial and political sanctions in place against Russia.
Mr Putin could be looking to use Ukraine as powerful leverage to get the US and other Western nations to lift or at least ease sanctions he believes are unjust.
If Russia launches an attack, the issue of Ukraine could be used in diplomatic discussions as a possible concession on these sanctions.
However, there is no guarantee this would work. An invasion of Ukraine could potentially lead to harsher economic and political repercussions from the West, as threatened by the UK over the weekend.
To cement access to a crucial sea port
Taking control of Crimea in 2014 gave Moscow continuing access to the naval base at Sevastopol, a warm water port and home to Russia's Black Sea Fleet.
Now, by seizing the belt of land between Russia and Transdniestria, which includes the Ukrainian cities of Mariupol, Kherson, and Odessa, Moscow could secure freshwater supplies for Crimea.
It would also block Ukraine's access to the sea, while avoiding major combat over Kyiv and Kharkiv.
This could potentially be part of a southern offensive by Moscow where it seizes only part of Ukraine, but an area with great strategic significance.
Could a pro-Moscow Ukrainian play a role?
According to British authorities, Russia is considering former Ukrainian politician Yevhen Murayev to head a pro-Russian leadership in Kyiv should an invasion take place.
Mr Murayev, who is also a prominent media owner, raised eyebrows in Ukraine when he said Russia's 2014 annexation of Crimea was good for his nation and should be recognised by the world.
But both the Russian Foreign Ministry and Mr Murayev himself have dismissed the British report as "disinformation".
Mr Murayev denied having any contact with the Kremlin and said the suggestion was "stupid", given he was placed under Russian sanctions in 2018.
Will the US send troops to Ukraine?
Not yet, although the nation has supported Ukraine with military equipment.
But President Joe Biden is carefully considering his options, according to The New York Times.
The Times reports that Mr Biden is pondering whether to send 1,000 to 5,000 troops to Eastern European countries and increase that number should the situation escalate.
"We are developing plans and we are consulting with allies to determine options moving forward," a US spokesperson said.
And Britain is also watching the situation, with Deputy Prime Minister Dominic Raab warning of "very serious consequences if Russia takes this move to try and invade."
