There had been talk of a possible invasion building for weeks, but the move has caused surprise and fear of what this will mean for the stability of Europe.
The West has condemned the move, with the Prime Minister Boris Johnston telling the nation in an address that, the UK "cannot and will not just look away" at Russia's "hideous and barbaric" attack.
Here is a simple explanation of why Russia has invaded Ukraine.
Why is Russia invading Ukraine?
Concerns about Russia invading Ukraine started late in 2021, when Russia began to station large numbers of troops near Ukraine's border.
Russia is unhappy that Ukraine has ambitions to join Nato, which President Putin sees as threatening Russia's, 'historic future as a nation'.
Russia has previously invaded Ukraine, most recently in 2014, when it annexed the Crimean peninsula after Ukraine’s pro-Russian president was removed through protests.
On February 21, Russia recognised the independence of the contested regions of Donetsk and started evacuating people to Russia.
Before announcing the invasion, Putin claimed that Russia was protecting people and invading the country with the aim for "the demilitarisation and denazification" of Ukraine.
However, Ukraine is a democratic country, with an elected government and poses no threat to Russia.
What has Boris Johnston said about Russia invading Ukraine?
Boris Johnston took to Twitter to share his feelings about the invasion early today, stating,
'I am appalled by the horrific events in Ukraine and I have spoken to President Zelenskyy to discuss next steps.'
'President Putin has chosen a path of bloodshed and destruction by launching this unprovoked attack on Ukraine. The UK and our allies will respond decisively.'
Mr Johnston addressed the UK at midday on Thursday, where he stated that Russia had began a "vast invasion by land, by sea and by air".
He commented that the UK would launch, a "massive package" of sanctions, but did not go into detail of exactly what would be imposed.
The Prime Minister has outlined what sanctions he plans to implement tonight in the House of Commons.
British Prime Minister Boris Johnson told the Commons he was imposing a series of measures.
All major Russian banks will have their assets frozen and they will be excluded from the UK financial system.
Laws to stop major Russian companies and the state from raising finance or borrowing money on UK markets.
Asset freezes on 100 new individuals or entities.
The Aeroflot airline will be banned from landing in the UK.
A suspension of dual use export licences to cover things which can be used for military purposes.
Within days the UK will stop exports of high-tech items and oil refinery equipment.
A limit on deposits Russians can make to UK bank accounts.
What has Ukraine said?
In an address to the nation, President Zelenskyy of Ukraine told Ukrainians, “We are defending our country, we fight for our country and we protect our country”.
Urging any Ukrainians who were able and willing to fight would be issued weapons.
What does the Russia Ukraine war mean for the UK?
There is no talk or plan for the UK or other allies to join forces with Ukraine.
The UK has already issued sanctions to Russia and plans to issue more, which will be outlined this evening by the Prime Minister.
However, there is growing concern about war in Europe and how this will destabilise the region.
There have already been stark price rises for oil, with a price of a barrel now the highest price since 2014 at $105 a barrel.
This means that people in the UK will be left with high prices at the petrol pump.
Ukraine is also known as the bread basket of Europe, so there is additional worry about how this could impact the cost of food.
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