Russia has launched a military operation in neighbouring Ukraine, moving to the outskirts Kyiv after unleashing air strikes on cities and military bases.
Tensions had escalated after Russia declared the independence of two rebel-backed regions of eastern Ukraine.
The crisis is complicated and exists in part due to deep ties between Russia and Ukraine, as well as resistance to the idea of Ukraine joining NATO, reports the Mirror.
Economic sanctions have been enforced by countries in the West in response to the escalating crisis.
More severe sanctions will follow now Russia has imposed what has been described as a 'full-scale invasion'.
Here we explain what Nato is, whether Ukraine is part of it and what the military alliance is likely to do next.
What is NATO?
NATO - the North Atlantic Treaty Organisation - is a military alliance of countries formed in 1949 that promise to come to each other's aid in the event of an armed attack.
It was originally formed in the wake of World War Two as the threat shifted to a possible armed struggle with the Soviet Union.
In response, the Soviets created their own military alliance under the Warsaw Pact of 1955, which included the likes of Poland, Romania and the Czech Republic, then Czechoslovakia.
Since the Soviet Union dissolved in 1991, the situation has become slightly more complicated, with the former Warsaw Pact states, Bulgaria, Albania, Romania, Czech Republic, Hungary, Germany (formerly East Germany) and Poland now all NATO members.
NATO's website said of its formation: "It is often said that the North Atlantic Treaty Organisation was founded in response to the threat posed by the Soviet Union. This is only partially true.
"In fact, the Alliance’s creation was part of a broader effort to serve three purposes: deterring Soviet expansionism, forbidding the revival of nationalist militarism in Europe through a strong North American presence on the continent, and encouraging European political integration."
Which countries are in NATO?
NATO was originally formed with just 12 members, but has since expanded to 30 states.
A full list of the countries currently part of NATO and the year they joined is as follows:
- 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)
- United Kingdom (1949)
- United States (1949)
Is Ukraine a member of Nato?
Ukraine is not part of Nato.
The country cooperates with NATO as a partner country of its Partnership for Peace programme, but it is not a member of the alliance.
Russia views expansion of the organisation as a threat, particularly on its own borders, which would be the case if Ukraine joined NATO.
Russian President Vladimir Putin fears that if Ukraine became a member, the alliance might try to recapture Crimea.
Mr Putin has demanded that Nato halt its expansion and withdraw troops or military equipment from countries neighbouring Russia.
This includes Ukraine, but also Nato members such as Estonia, Latvia and Lithuania.
Nato general secretary Jens Stoltenberg has said the “door remains open” for Ukraine to join the alliance.
What will Nato do now Russia has invaded Ukraine?
Because Ukraine is not a member of Nato, member countries are not obliged to play a direct role in combat in Ukraine.
However, Nato secretary-general Jens Stoltenberg has warned that the military alliance would defend every inch of its territory should Russia attack a member country.
Speaking after chairing an emergency meeting of Nato envoys, Mr Stoltenberg said the 30-nation security alliance will continue to beef up its defences on its eastern flank near Ukraine and Russia.
European Union and Nato member Lithuania has declared a state of emergency since the Baltic nation borders Russia’s Kaliningrad region to the south west and Russia’s ally Belarus to the east.
Nato countries have reportedly got 100 jets and 120 ships on high alert as a deterrent.
Western allies have said for weeks that they would not send troops into the country.
The goal instead is to make Moscow pay through economic sanctions in a bid to make the Kremlin change course.
