Since Russia's invasion of Ukraine, Finland and Sweden have moved to apply for membership of the North Atlantic Treaty Organisation (NATO) military alliance, which would mark a major policy shift for the Nordic region.
NATO Secretary-General Jens Stoltenberg has said the countries could join "quickly" and that he was sure arrangements could be found for the interim period.
But Russia has repeatedly warned of "serious consequences" if Finland and Sweden join NATO, saying it would have to strengthen its land, naval and air forces in the Baltic Sea, and raised the possibility of deploying nuclear weapons in the area.
Britain has pledged to come to the aid of Sweden and Finland, including with military support, if the two Nordic nations came under attack under security deals Prime Minister Boris Johnson signed on Wednesday with his Swedish counterpart in Stockholm and the Finnish president in Helsinki.
Here's what you need to know about the Western military alliance and why it could expand in response to Russia's attack on Ukraine.
What is NATO?
NATO is a European and North American defence alliance set up to promote peace and stability and to safeguard the security of its members. It was created in 1949 as the Cold War escalated and is headquartered in Brussels, Belgium.
Its aim was to provide collective security against the Soviet Union and Eastern Europe's communist-run countries.
Russia reacted by setting up its own military alliance called the Warsaw Pact.
Which countries are in NATO?
Twelve founding countries - the United States, Canada, the United Kingdom, France and eight other European nations - signed the North Atlantic Treaty in 1949, pledging to protect each other by political and military means.
Over the decades since, the alliance has grown to include a total of 30 members.
In alphabetical order, they are: Albania, Belgium, Bulgaria, Canada, Croatia, Czech Republic, Denmark, Estonia, France, Germany, Greece, Hungary, Iceland, Italy, Latvia, Lithuania, Luxemburg, Montenegro, the Netherlands, North Macedonia, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Turkey, the UK and the US.
Ukraine is not a member of NATO, but has long hoped to join the alliance. This is a sore point for Russia, which sees NATO as a threat and vehemently opposes the move.
How does NATO work?
Members agree to help one and another in the case of an armed attack against any individual member state, a principal known as Article 5.
As the bloc says: "An attack against one Ally is considered as an attack against all Allies."
This is crucial for many of the smaller countries who would be defenceless without its allies. Iceland, for example, has no standing army.
Since the US is the largest and most powerful NATO member, providing the bulk of forces and budget, any state in the alliance is effectively under US protection.
Who runs NATO?
The alliance's meetings — the North Atlantic Council, held at ambassadorial level almost weekly in Brussels, less often at the level of ministers or heads of state and government — are chaired by NATO Secretary-General Jens Stoltenberg, a Norwegian politician.
But the US is without doubt the biggest and most influential member. It spends more on its own military budget than all the others combined. It also pays just over 22 per cent of NATO common funding for infrastructure and collectively owned equipment. So Washington has a big say in how things are run.
In essence, Stoltenberg runs the headquarters located near Brussels.
He doesn't order the allies around. His job is to encourage consensus and speak on their behalf publicly as a single voice representing all 30 members.
Why does NATO worry Russia?
The stated reason is that a further eastward expansion of NATO would pose a security threat to Russia. Washington and its allies deny this is a valid worry, since no NATO country is threatening to use force against Russia.
More broadly, Putin wants NATO to pull back its existing military presence in Eastern Europe, which includes a regularly rotating series of exercises in Lithuania, Latvia and Estonia, all former Soviet states.
Putin also opposes NATO's missile defence presence in Romania, a former Soviet satellite state, and a similar base under development in Poland, saying they could be converted to offensive weapons capable of threatening Russia.
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What does it mean if Sweden and Finland join?
While both countries have small but capable armed forces, geography also plays a major part in Moscow's concerns.
Russia and Finland share a 1300km border; the Kola Peninsula is a "strategic bastion" Moscow considers key for Russia's national security.
It is also the home of the Russian Northern Fleet, and Russia's second-biggest city, St Petersburg, lies some 170km from the border with Finland.
The addition of Finland would be quite a boon for NATO. Despite its relatively small population, Finland is a serious military power that has been unofficially aligned with the West for decades.
Its military has for decades used equipment purchased from the US that is compatible with NATO allies, meaning it could easily join NATO missions should it choose to do so.
How is NATO helping Ukraine?
Since the invasion of Ukraine, the military alliance has deployed forces in countries along NATO'S eastern flank in Estonia, Latvia, Lithuania and Poland.
These units are prepared for combat and are led by Britain, Canada, Germany and the US, respectively.
The alliance has also sent planes and ships to NATO countries in eastern and south-eastern Europe.
NATO has also been providing large supplies of weapons and equipment to the Ukraine military including anti-tank missiles, armoured vehicles and anti-aircraft weapons.
It s also supplying Ukraine with valuable intelligence reports about Russian troop strengths and deployments.
NATO does not have any troops in Ukraine, and no plans have been announced to send troops from the alliance into the country.
The alliance also ruled out imposing a no-fly zone over Ukraine.
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- Reported with CNN, Associated Press
