Ukraine conflict: What is Nato and how is it changing?

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Finland and Sweden already have highly capable militaries

Sweden and Finland have applied to join the Western security alliance Nato, because of the war in Ukraine.

Russia's invasion has also prompted Nato to announce plans to increase the number of troops that can be deployed at short notice.

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.

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How could Finland and Sweden join Nato?

It can take a year to apply and become a Nato member, and all member states must agree that a new country can join.

Most members are keen for Finland and Sweden - which have been neutral for many years - to join. But Turkey is against their membership - 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.

Nato members 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".

How else is Nato changing?

Nato has announced plans to massively increase the number of its forces at high readiness from 40,000 to over 300,000 troops.

Its secretary general Jens Stoltenberg described this as "the biggest overhaul of our collective deterrence and defence since the Cold War".

Is Nato's Nordic expansion a threat or boost to Europe?

The rapid reaction force is a combination of land, sea and air assets designed to be deployed quickly in the event of attack. Some of these have already been sent to countries bordering Russia and Ukraine.

Since Russia's invasion, Nato has also increased the number and size of its multi-national battle groups stationed in eastern Europe.

At a leaders' summit in Madrid, Nato members are also expected to change the alliance's official stance towards Russia, which was adopted in 2010 and described Moscow as a "strategic partner".

Mr Stoltenberg said he expected Russia to instead be described as posing "a direct threat to our security, to our values, to the rules-based international order".

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.

Nato has stepped up its military defences in eastern Europe

This is also why Nato rejected a no-fly zone over Ukraine.

Individual countries including the UK and the US are providing weapons to Ukraine, such as missiles and armoured vehicles.

What weapons are being given to Ukraine by the UK?

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.

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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."