Finland and Sweden already have highly capable militaries
Finland has said it wants to join Nato and neighbouring Sweden is expected to make a decision soon.
The move is supported by existing Nato countries, who are supplying Ukraine with weapons to fight Russia’s invasion.
What is Nato?
Nato – the North Atlantic Treaty Organization – is a military alliance. It was formed in 1949 by 12 countries, including the US, UK, Canada and France.
Members agree to come to one another’s aid if they come under armed attack. Finland will take it to 31 members.
Nato’s original aim was to counter Russian expansion in Europe after World War Two.
Following the Soviet Union’s collapse in 1991, many of its former Eastern European allies joined Nato.
How could Finland and Sweden join Nato?
Both countries have been neutral for many years, but public support for joining Nato has grown since Russia invaded Ukraine.
It can take a year to apply and become a Nato member. But the UK and US have said they would offer military support to Sweden and Finland if they were attacked beforehand.
In order to join Nato, countries must be democracies, treat minorities fairly and commit to resolving conflicts peacefully.
They must also provide military support to the alliance.
Both Finland – which has a 1,340km (830 mile) border with Russia – and Sweden have highly capable militaries.