Russia's President Vladimir Putin has launched a full-scale invasion of Ukraine, with reports of missile strikes on major cities and military convoys crossing the border.
In a televised address, Putin warned that Moscow's response would be "instant" if anyone tried to take on Russia. Ukraine's foreign minister has accused Russia of starting a full-scale war and called on the UK to do "everything possible" to stop it.
United States President Joe Biden said the world will hold Russia accountable and UK Prime Minister Boris Johnson pledged a decisive response from the UK and its allies.
With Ukrainian capital Kyiv already rocking from explosions, the military reporting an air attack by Russia and the first deaths being reported, we've looked at the causes of the conflict and what Russia wants.
What is the cause of the tension in the region?
The current difficulties date back to the overthrow in 2014 of the pro-Moscow Ukrainian government of Viktor Yanukovych, prompting fears in the Kremlin that the country was moving into the orbit of the West.
President Vladimir Putin responded by sending in troops to annex Crimea while Russian-backed separatist rebels seized territory in eastern Ukraine in bloody fighting with the Ukrainian military.
Why does Putin say Ukraine isn't a country?
In a public statement on Monday night, ahead of declaring two parts of Ukraine as separate states, Russian premier Vladimir Putin repeated a long-held Russian belief that the Ukraine is not a real country.
The notion that Ukraine is not a country in its own right, but a historical part of Russia, is 'deeply ingrained in the minds of many in the Russian leadership', according to Dr Björn Alexander Düben, Assistant Professor at the School of International and Public Affairs at the London School of Economics.
Dr Düben explained that Russia and Ukraine have the same ancient roots. He said: "Ukraine’s sentimental and spiritual appeal to many Russians derives from the fact that the Kievan Rus’ – a medieval state that came into existence in the 9 th century and was centred around present-day Kiev – is regarded as a joint ancestral homeland that laid the foundations for both modern Russia and Ukraine."
But he added that from the beginning, the Rus' was fragmented, occupied invaded and divided by a series of forces - including the Mongols, Poland and Lithuania.
From the 14th century and for 400 years what is now Ukraine was ruled by Poland and Lithuania, with a culture and language distinct from Russia.
Following the Partitions of Poland in 1790 Russia absorbed much of what is now modern-day Ukraine - a state that remained for 120 years.
When the Russian Empire collapsed in 1917 Ukraine declared itself a state of its own - but it quickly became part of the Soviet Union as a founder member of the USSR in 1922 - until the Union itself collapsed in 1991, and Ukraine was again an independent country.
Has the Crimea always been fought over - and what has happened before?
The history of Crimea, part of Ukraine that was annexed by Russia in 2014, has been marked by fighting and conquest.
From the 5th century BC and for almost 2,000 years it was a Greek settlement, then part of the Roman Empire, the Byzantine Empire, the Empire of Trebizond and then fought over by nations including the Venetians and the Genovese.
It was acquired by the Kievan Rus becoming part of what is modern-day Ukraine in the medieval period, but was then conquered by the Golden Horde, the Khanata and the Ottoman Empire.
In 1774 the Ottomans were defeated by Russia and traded Crimea to Catherine the Great. Crimea was annexed into Russia in 1783.
The Crimean War was a conflict between 1853 and 1856 in which Russia fought an alliance of the UK, France, the Ottoman Empire and Sardinia - partially over Russian expansion. The war included the famous Charge of the Light Brigade and was the conflict which made Florence Nightingale famous.
It was an independent Soviet state from 1921 to 1945, then a province of Russia from 1945 to 1954 and then a province of Ukraine - until it was annexed by Russia in 2014.
Why are there two separatist zones?
Mr Putin has decided to recognise the two Russian-backed regions in east Ukraine – Donetsk People’s Republic and Luhansk People’s Republic – as independent states.
People waving Russian flags in Donetsk (Alexei Alexandrov/AP)
He has ordered troops into the two regions to “maintain peace” – crossing the red line set by the West by deploying Russian forces on Ukrainian territory.
Russian-backed forces have held parts of the regions since Crimea was annexed in 2014. A pact between Moscow and Kyiv was signed in 2015 - giving the regions a large degree of autonomy from Ukraine.
But that agreement, Minsk 2, has never been implemented to Putin's satisfaction - he wants more control over the regions and the policy of Ukraine as a whole.
What do the people who live there want?
Across Ukraine the population has long been divided between those who look towards Russia, and those who look towards an independent Ukraine.
Controlling forces in Crimea, Donetsk and Luhansk may prefer Russian 'control' but the majority of the people want a united, free, peaceful Ukraine.
According to IPI Global Observatory most citizens 'strongly feel' that they are Ukrainian.
What does Putin want to achieve?
According to The Guardian, Putin is said to want to rebuild a Russian sphere of influence in eastern Europe, embracing former Soviet republics such as Estonia, Latvia, Lithuania, Belarus, Georgia and Ukraine.
Putin may also hope to demonstrate that Russia is still a superpower.
Putin fears Ukraine, which borders Russia, is growing too close to the West and Nato, as well as the EU - a free and democratic Ukraine would be a 'bad example' to the citizens of Russia.
In short he is attempting to rebuild what was the Soviet Union.
What would the west do if Russia does mount a full invasion?
The UK, European Union and United States will all impose economic sanctions.
Boris Johnson said the measures being imposed by the UK were the “first barrage” because further action would be taken if the situation escalated.
The West will hope the economic pressure it can impose through sanctions will persuade Mr Putin to back down.
But he has an estimated 150,000 troops ringed around Ukraine and Mr Johnson warned “all the evidence is that President Putin is indeed bent on a full-scale invasion of the Ukraine”.
An attempt to conquer Ukraine could lead to a prolonged and bloody conflict.
Ukraine is not a member of Nato, so the alliance’s members – including the UK – will not play a direct role in fighting.
The UK has supplied anti-tank weapons to the Ukrainian army and other support is being considered.
But Russian aggression in eastern Europe is likely to increase anxiety among Nato’s members.
This could lead to a further build-up of forces, potentially increasing tensions even further in the region.
What is the advice for Britons in Ukraine?
They have already been told to leave while it is still possible to do so.
The British ambassador in Ukraine Melinda Simmons has relocated from the embassy in Kyiv to Lviv in the country’s west due to fears about the risk posed to the capital from an invasion.
In a message on Tuesday she said: “I strongly advise British nationals to leave the country while commercial travel is available.”
