The West has accused Vladimir Putin of launching an invasion of Ukraine after the Russian president ordered soldiers into two breakaway regions.
Putin claimed his troops were on a "peacekeeping" mission and that he recognised the Donetsk and Luhansk areas in eastern Ukraine as independent. But Western countries said the move was an attempt to create a pretext for a further invasion, warning it could be the biggest war in Europe since 1945.
It comes after weeks of increasing tension that has pitted Russia against Nato allies, including the US and UK, who have repeatedly told Putin to back down.
The escalation has put renewed focus on the 73-year-old transatlantic military alliance, which in recent years has struggled to maintain the same level of relevancy that it had in the latter decades of the 20th century.
As recently as 2019, it was a fractured organisation with French president Emmanuel Macron bemoaning its "brain death" in the light of a Trump-led US focus on budgets rather than geo-political importance.
Yahoo News UK explains what Nato is, why Putin regards it as a foe, and outlines the unique defensive pact that binds its members together.
Watch: Russia approves use of armed forces as NATO warns of 'full-scale attack'
What is Nato?
The North Atlantic Treaty Organization is a political and military alliance of 30 countries.
Nato was set up in 1949 to protect members against the Soviet Union, with 12 nations initially signing up to the North Atlantic Treaty in Washington DC.
These countries were the US, Canada, the UK, Belgium, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway and Portugal.
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Nato's political objectives are to "promote democratic values", "enable members to consult and cooperate on defence and security-related issues", and to "prevent conflict".
The organisation said it "is committed to the peaceful resolution of disputes", but if diplomacy fails it will use its military power "to undertake crisis-management operations".
Nato has four multinational battalion-size battlegroups, or some 4,000 soldiers, led by Canada, Germany, the UK and the US in Latvia, Lithuania, Estonia and Poland.
The troops serve as a "tripwire" for Nato's 40,000-strong response force to come in quickly and bring more US troops and weapons from across the Atlantic.
Which countries are in Nato, and what date did they join?
1949: Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the UK, the US.
1952: Greece, Turkey
1999: Czech Republic, Hungary, Poland
2004: Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia, Slovenia
2009: Albania, Croatia
2020: North Macedonia
Why does Putin see Nato as a threat?
Since the Cold War ended, Nato has expanded eastwards by taking in 14 new countries, including the states of the former Warsaw Pact and the three Baltic nations that were once in the Soviet Union.
Russia sees this as a threatening encroachment towards its borders and continues to say it was a betrayal of Western promises at the start of the 1990s – something Nato denies.
Ukraine is not a Nato member but has a promise dating from 2008 that it will eventually get to join.
Since toppling a pro-Russian president in 2014, Ukraine has become closer politically to the West, staged joint military exercises with Nato and taken delivery of weapons.
Kyiv and Washington saw these as legitimate moves to bolster Ukraine's defence after Russia seized the Crimea region in 2014 and provided backing to separatists in eastern Ukraine.
Putin believes Ukraine's growing ties with the alliance could make it a launchpad for Nato missiles targeted at Russia.
He said Russia needs to lay down "red lines" to prevent that.
In an address on Monday, Putin said: "In Nato documents, our country is officially and directly declared the main threat to North Atlantic security. And Ukraine will serve as a forward springboard for the strike."
But his demands that Ukraine drop its long-term goal of joining the Atlantic military alliance have been repeatedly rebuffed by Kyiv and Nato states.
What happens if a Nato country is attacked?
The collective defence clause of Nato's founding treaty – Article 5 of the Washington Treaty – is a provision that means an attack against one member is considered an attack against all of them.
This is a fundamental part of Nato and why it says it is a defensive alliance.
Nato says military operations are carried out under Article 5 or a United Nations mandate, alone or in co-operation with other countries and international organisations.
Ukraine is not a member of Nato and the alliance is not treaty-bound to defend it.
US president Joe Biden has said he will not send American or allied troops to fight Russia in Ukraine.
However, Kyiv is a close partner and was promised eventual membership of the alliance.
The 30-member Nato works with Ukraine to modernise its armed forces.