Russian president Vladimir Putin's decision to order troops into eastern Ukraine has escalated tensions that have long simmered between the nations.
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In signing a decree "recognising" two Russian-backed separatist territories in Ukraine, the so-called Donetsk and Luhansk People's Republics, Mr Putin drew condemnation for violating international law.
What Russia does next, and how NATO and other Western nations respond, could have far-reaching consequences for the international order, and threaten to reverberate in Australia's own region, experts say.
Why has Putin ordered Russian troops into eastern Ukraine?
Mr Putin's claim that the military presence in eastern Ukraine will be for "peacekeeping" has been roundly and swiftly dismissed as nonsense. World and Australian leaders, and international affairs experts, have been unequivocal that the move violates Ukrainian territory.
Australian National University international relations and security expert, John Blaxland, says Mr Putin has acted unilaterally in signing the decrees "recognising" the separatist territories and ordering troops into Ukraine.
"He calls them peacekeepers. But these are about enforcement, this is about enforcing his will. This isn't keeping the peace," Professor Blaxland says.
Sonia Mycak, an expert on the history of conflict between Ukraine and Russia, says there are two main reasons driving Mr Putin's actions. One is an imperialistic ambitions to take over Ukraine and re-establish a Russian empire, motives that stretch back hundreds of years to tsarist rule and later the Soviet Union.
Mr Putin does not accept Ukrainian statehood and says it is a country created by Russia.
The second motive, Dr Mycak says, is that Russia does not recognise that Ukrainians are a separate people with their own distinct language, culture and identity.
The situation is not complicated, she says.
"This is a large, aggressive neighbour that wants to take the territory and the property of a neighbour by brute force," Dr Mycak says.
Director of the Defence, Strategy and National Security program at the Australian Strategic Policy Institute, Michael Shoebridge, says Mr Putin is also attempting to change Russia's sphere of influence in Europe.
"He's determined to establish a new security construct in Europe, where Russia dictates the choices of the countries that border Russia, and where the security structure that has been there since the end of the Second World War is dismantled," Mr Shoebridge says.
"This is not just about Ukraine, but about the other countries that border Russia, and how the rest of Europe relates to them."
Will this lead to war?
A Russian invasion of Ukraine would be the largest military action in Europe since World War II and could lead to large-scale conflict and deaths.
Ukraine has stressed that it wants to reach a diplomatic solution to the crisis, and will not respond to Mr Putin's latest moves by taking military action in the separatist territories, which it believes could give Russia a pre-text to invade.
While the latest escalation in tensions may appear as new to Western nations, the conflict has been ongoing while international attention has been elsewhere.
Dr Mycak says in fact Ukraine has been at war with Russia since Russia occupied and then annexed Crimea, and backed separatists in Donetsk and Luhansk in 2014.
There have already been 15,000 Ukrainian soldiers and civilians killed in the conflict, she says.
What does this mean for Australia?
While the conflict may seem a long way from Australia geographically, a Russian invasion in Ukraine has serious potential consequences for Australians and the Indo-Pacific region.
One is that it would set a precedent that a nation can unilaterally decide to invade another nation's territory and acquire it by military force, Professor Blaxland says.
"This is really lowering the bar of what is acceptable internationally. And the repercussions are actually hard to fathom, but they're not good," he says.
How NATO responds to Russia in Ukraine could influence China's actions on Taiwan, national security experts say.
Professor Blaxland says if Western nations react lamely to a Russian invasion of Ukraine, Chinese president Xi Jinping may be emboldened over Taiwan.
The Chinese and Russian presidents met earlier this month on the sidelines of the Beijing Winter Olympics, and issued a statement affirming their support for each other's foreign policy - including Russia's backing of China over Taiwan, and China's opposition to "further expansion of NATO".
Mr Shoebridge says the two authoritarian actors were seeking spheres of influence letting them dictate the choices of countries in their region. It made them close partners with a common agenda, and meant that the situation in Ukraine was directly relevant to the Indo-Pacific.
"This is now a coming together, there's a common strategic challenge that European democracies and Indo-Pacific democracies face," he says.
"We don't get a choice about taking an interest in the wider world and just focusing here at home."
Professor Blaxland says the world is more connected today than ever.
"In the past, we've thought conflicts on the other side of the world don't matter so much, perhaps. But in this day and age, what happens in Ukraine has global ramifications," he says.
What should Australia, and NATO countries, do in response?
Australia has already offered to assist Ukraine on cyber security, and already has sanctions targeting Russia in place.
Dr Mycak says more sanctions targeting Russia's powerful oligarchs, who live luxurious lifestyles in Western countries, would put pressure on Mr Putin and could prevent him annexing the territories in eastern Ukraine.
She also says it is not too late for a diplomatic solution, but that Western nations must act swiftly.
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Prime Minister Scott Morrison has ruled out Australian military action, and Professor Blaxland says Australia is not in a position to do much militarily beyond cybersecurity support.
Australia needs to work closely with NATO in its response, Professor Blaxland says.
"We must be eager for this precedent not to go unchallenged," he says.
Russia's actions should also awaken Australia to the fact that using military force to achieve aims is on the minds of leaders like Mr Putin and Mr Xi, according to Mr Shoebridge. They also showed the importance of the Quad relationship with the US, India and Japan, and the new AUKUS alliance.
"Realising that truth means that the urgency of the Quad and AUKUS agenda is something we need to take to heart and act accordingly."