Russian president Vladimir Putin has warned that Moscow’s response will be “instant” if anyone tries to take on his nation.

There are fears this could be the worst conflict seen in Europe since the Second World War.

Ukraine has said this amounts to full-scale war, and the US, UK, UN, EU and North Atlantic Treaty Organisation (Nato) have promised to stand by Ukraine and do what they can to protect their ally.

READ MORE: Russia launches 'full-scale' invasion of Ukraine

But why is Russia invading Ukraine and what is Nato?

Why is Russia invading Ukraine?

Putin has accused the west of lying to Russia, promising in the 1990s that Nato would not “move an inch to the east”.

He has accused Nato of provoking Russia in its move eastwards, which Russia sees as a threat to its sovereignty and as a provocation.

After the cold war ended and the USSR disintegrated, many countries which had been under Soviet Union influence, aligned themselves with Nato and the west.

Eastern European countries such as Poland, Latvia and Romania chose to join the alliance.

Russia has launched a war on Ukraine, with missile strikes and explosions heard in major cities

This meant Nato was significantly closer to Russia than it had been in the past - removing Russia's eastern-European buffer zone with the military bloc.

Ukraine has signalled its willingness to join the alliance, which would mean Nato had a large border with Russia.

Five Nato countries currently border Russia with many hosting US military bases.

However, western countries have slapped down the idea it is about Nato expansion and more about bringing Ukraine back under Russian control - and about Putin's legacy.

During his speech where he announced he would send troops into eastern Ukraine, the Russian president denied that Ukraine was ever a real country.

What is Nato?

Nato, also known as the North Atlantic Alliance, was founded after the Second World War with the aim of securing peace in Europe and countering the threat of the Soviet Union.

Its founding treaty was signed in the US in 1949 by a dozen European and North American countries, including the UK.

Russia has been furious over Nato's expansion eastwards

One of the most important aspects of Nato memberships is that of collective defence (commonly known as article 5) meaning an attack against one member would be considered an attack on all members.

Since its founding, Nato has grown from its original 12 members to 30.

READ MORE: Nicola Sturgeon says Russia must face 'severest consequences' for invasion of Ukraine

Many countries now in Nato are former Warsaw Pact nations, a treaty signed in 1955 in which Russia promised military support to Soviet satellite states.

The military and political alliance says it has helped keep peace throughout Europe since it was founded.

There are 3.6 million active personally and some 3.7m inactive reserves throughout countries in the organisation.

And Nato member states account for most of the world’s military spending - around $1 trillion. America alone accounted for about wo-thirds of the alliance’s budget.

Is Ukraine a Nato member?

No, but it would like to be. Along with Georgia, Ukraine has been trying to join Nato for years. In 2008, the countries began the formal process to begin a Nato Membership Action Plan.

However, this plan was shut down by president Viktor Yanukovych in 2010 when he was elected as leader. 

But Yanukovych himself was forced to flee the country in 2014 following widely held protests.

Ukraine's current leader, Volodymyr Zelenskyy, is in favour of joining the military alliance, reportedly asking Joe Biden when he first became president "why are we not in Nato yet?"

Support for Ukraine joining the bloc has increased since Russia annexed Crimea in 2014 and Nato has said the nation will eventually become a full member of the alliance.

This has not gone down well in Russia, with president Putin previously asking for assurances that Ukraine wouldn't join Nato - continuing the Kremlin's claim that Nato is breaking its promise not to expand further east. 

Which countries are Nato members and when did they join?

Albania (2009)

Belgium (1949)

Bulgaria (2004)

Canada (1949)

Croatia (2009)

Czech Republic (1999)

Denmark (1949)

Estonia (2004)

France (1949)

Germany (1955)

Greece (1952)

Hungary (1999)

Iceland (1949)

Italy (1949)

Latvia (2004)

Lithuania (2004)

Luxembourg (1949)

Montenegro (2017)

Netherlands (1949)

North Macedonia (2020)

Norway (1949)

Poland (1999)

Portugal (1949)

Romania (2004)

Slovakia (2004)

Slovenia (2004)

Spain (1982)

Turkey (1952)

The United Kingdom (1949)

The United States (1949)