An all-out invasion of Ukraine is under way after Russian President Vladimir Putin ordered a special military operation to support separatists in eastern Ukraine's Donetsk and Luhansk regions.
While Mr Putin said the operation was intended to topple the Ukrainian government and demilitarise the country, he denied having plans to occupy Ukraine.
Here is what you need to know about the crisis:
Once a part of the Soviet Union, Ukraine became an independent country with the collapse of the state in 1991. A nation of 44.1 million people, about 43 per cent to 46 per cent of Ukrainians converse in the Russian language despite the separation, and a sizeable segment of the population is pro-Russian.
While Russia has maintained close relationships with most of the former Soviet states that broke away from it, ties with Ukraine have been frosty since a pro-Western revolution in 2014 ushered in politicians inimical to Moscow's interests.
The revolution - which was sparked by the suspension of an association agreement with the European Union - prompted Russia to invade and annex the Crimean Peninsula in the eastern part of Ukraine, which is made up mostly of ethnic Russians.
On Monday (Feb 21), Mr Putin further upped the ante by recognising the independence of eastern Ukraine's Donetsk and Luhansk separatist republics, even in areas that were not under their control, despite warnings from the West not to do so.
The announcement was made during an hour-long speech laden with historical references in which Mr Putin questioned Ukraine’s right to exist, arguing that the country was “entirely created by Russia”.
"To put it simply, Russia just announced that it is carving out a big chunk of Ukraine," United States President Joe Biden said. "This is the beginning of a Russian invasion."
Russia has claimed its actions are influenced by Ukraine's ambitions to join the US-led North Atlantic Treaty Organisation, which Moscow sees as a red line that would put the alliance's missiles next door.
The move to recognise the separatists prompted countries such as the US, Britain, Australia, Canada and Japan, besides the EU, to impose sanctions targeting Russian banks and elite members of Mr Putin's inner circle. The Nordstream 2 gas pipeline project from Russia to Germany was also halted in an effort to inflict financial pain on Russia.
But a defiant Mr Putin gave the nod on Thursday for assaults on military facilities across Ukraine, including airfields and anti-aircraft systems, as well as the bombardment of capital Kyiv and other cities. That has provoked even more punishing sanctions that will affect Russia's ability to access the global financial system, as well as key technologies.
Why it matters?
A war between Russia on one side and Ukraine and its Western allies on the other could have a catastrophic impact not just in terms of lives lost and destruction, but also on a global economy yet to recover from the Covid-19 pandemic.
Oil and gas are the main exports of the Russian economy, and there have already been warnings of gas prices rising in Europe following the imposition of sanctions.
Meanwhile, Ukraine is known for holding the second-largest gas deposits in Europe, though they are largely unexploited. Crude oil prices have crossed US$100 a barrel - their highest level since 2014 - amid the uncertainty.
In addition, both Russia and Ukraine are key suppliers of grain and edible oils, resulting in global food prices shooting up due to the geopolitical uncertainty.
Casualties, meanwhile, are beginning to tick upward, with Ukraine announcing on the first day of the invasion that at least 50 people had been killed. Dozens of Russian fatalities are believed to have occurred.
There have been ballistic missile strikes on Kyiv and other population centres, with people urged to go to shelters. With Russian ground forces now well inside Ukraine's border, fighting has also escalated and there have been reports of Russian military planes and helicopters being shot down.
The war is asymmetric, with Russia's massive army more than capable of overwhelming Ukraine's smaller military despite the latter's upgrades in recent years and aid from Western backers.
Nato has clarified that it will not be sending troops into Ukraine, dealing a blow to Ukrainian hopes of more international involvement, but the indirect support of the US and its allies could make the conflict a drawn out affair.
For the time being, it looks like diplomatic efforts to resolve the crisis have failed, with negotiations called off after the belligerent Russian moves.
Whether there is still scope for a climbdown by the parties involved remains to be seen, but chances are looking slim that cooler heads will prevail in the near term as Russian forces move to take Kyiv.