Sweden and Finland have confirmed they will apply to join the Western security alliance Nato.
Their decision was prompted by Russia's invasion of Ukraine. It is being welcomed by most of the 30 existing countries in Nato.
However, Turkey is opposing either country joining the pact, saying the two countries support anti-Turkish terrorists.
What is Nato?
Nato - the North Atlantic Treaty Organization - is a defensive 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, and all member states must agree that a new country can join.
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.
Nato members agree to spend 2% of their GDP on defence. Finland already meets this target and Sweden says it will do so "as soon as possible".
However, one Nato member, Turkey, says it will refuse to let in either Sweden or Finland.
Turkey's president, Recep Tayyip Erdogan, says the two countries have been harbouring members of the Kurdistan Workers' Party (PKK), a group it views as a terrorist organisation.
Mr Erdogan also says they are harbouring followers of Fethullah Gulen, who he accuses of orchestrating a 2016 coup attempt.
Why isn't Nato sending troops to Ukraine?
As Ukraine isn't a member, Nato isn't obliged to come to its defence.
Nato countries fear that if their troops confront Russian forces, it could lead to an all-out conflict between Russia and the West.
This is also why Nato rejected a no-fly zone over Ukraine.
Why is Russia opposed to Nato?
Nato offered Ukraine a path towards membership in 2008. After Russia's annexation of Crimea in 2014, Ukraine made joining a priority.
But this has not happened, mainly because of Russia's long-standing opposition.
Russia believes Nato has been encroaching on its area of political influence by accepting new members from eastern Europe - and admitting Ukraine would bring Nato into its backyard.
Ukraine's President Zelensky has accepted his country can't join Nato at present, saying: "It is clear that Ukraine is not a member of Nato. We understand this."
Russia has warned Finland and Sweden against joining.
Which weapons have the UK and other countries sent to Ukraine?
Initially, Nato countries limited their supplies to defensive weapons.
The UK and US sent thousands of anti-tank missiles and anti-aircraft missiles.
Slovakia sent its S-300 anti-aircraft defence system, which can destroy aircraft up to 400km away. The US and Turkey have both sent drones armed with missiles.
Several Nato countries are now sending heavier weapons, to enable Ukraine to counter-attack Russia's army.
The US is sending helicopters, artillery and armoured personnel carriers. It plans to offer a $33bn (£26bn) grant for security and economic assistance to Ukraine on top of the $3.7bn (£3bn) it has already spent.
The UK is offering an extra £300m in military support to Ukraine and plans to send armoured vehicles, electronic warfare equipment and anti-artillery radar systems.
The EU has said it will spend up to 450m euros (£376m) to fund the supply of weapons to Ukraine - the first time the EU has helped provide arms to a warzone.
How many troops does Nato have in Eastern Europe?
Nato already had troops stretching from the Baltic in the north to Romania in the south.
They had been stationed there in 2014, after Russia's annexation of Crimea.
Nato has now sent elements of its 40,000-strong Response Force to countries bordering Russia and Ukraine.
It has fighter jets on alert and naval ships, including aircraft carrier groups, patrolling the seas.