This invasion comes after Russian demands to block Ukraine joining NATO were rejected by the defence alliance, who stated any independent country is free to join.
But what is NATO and why doesn't Russia want Ukraine to join?
Below is everything you need to know about NATO.
What is NATO?
NATO's aim is to offer collective security to all of its members.
This means that if a member of NATO is threatened by an external country, the other countries in NATO will help defend them.
NATO intervention has previously been seen in Bosnia, Herzegovina, Kosovo and Libya.
What does NATO stand for?
NATO stands for the North Atlantic Treaty Organisation and was established after World War Two.
The organisation started in 1947 when France and the United Kingdom signed the Treaty of Dunkirk to create an alliance in the event of a German or Soviet attack.
The treaty was then expanded to include more countries in the North Atlantic Treaty - this was signed in 1949.
Which countries are in NATO?
There are currently 30 member states of NATO, as follows:
Why does Russia want to stop Ukraine joining NATO?
No - Russia is against NATO and does not want Ukraine to join the alliance either.
Russia and NATO have had a historically complex relationship, and Russia does not want Ukraine joining NATO as it sees the organisation as a negative Western influence.
Russia also fears that if Ukraine joins NATO, Western-influenced countries will then be at its borders.
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