Sweden and Finland have applied to join the Western security alliance Nato, because of the war in Ukraine.
Most of the 30 existing Nato countries are keen for them to join, although Turkey is against their membership and says they 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. Finland and Sweden would take it to 32 members.
Members agree to help one another if they come under armed attack.
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.
However, 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.
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".
Turkey, says it will refuse to let in either Sweden or Finland. Its 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.
However, the US says it is confident Finland and Sweden will both join, despite Turkish objections.
What weapons are the US and UK sending to Ukraine?
The UK is sending long-range missiles to Ukraine, fired from an M270 multiple-launch rocket system.
It can launch 12 surface-to-surface missiles within a minute and strike targets up to 50 miles (80km) away.
The US has said it will send Ukraine a similar weapon - the M142 High Mobility Artillery Rocket System.
Russia has protested, saying the rockets could hit targets within Russia. It has threatened to expand the list of its targets in Ukraine in response.
However, the US has told Ukrainian forces only to fire the rockets against targets in Ukrainian territory.
The UK and US have also sent thousands of anti-tank missiles and anti-aircraft missiles.
They and other Nato countries are also sending drones, artillery, armoured vehicles and electronic warfare equipment.
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 thinks that 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."
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.