The North Atlantic Treaty Organisation, known as Nato, is a military alliance made up of nations across Europe and North America.
Formed in the aftermath of the Second World War in 1949, Nato's original goals were to secure peace in Europe, promote co-operation among its members and counter the threat posed by the Soviet Union.
Every year on 4 April, the anniversary is celebrated of the signing of Nato's founding document, the Washington Treaty, in 1949.
The 74th anniversary in 2023 is particularly historic, with Finland becoming the 31st member of the alliance.
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Beginning with 12 member states, Nato has now grown to 31, with North Macedonia joining in 2021 and Finland in 2023, following the Russian invasion of Ukraine in February 2022. Sweden, which applied to join Nato last May, is still waiting to join the alliance.
Sweden and Finland previously followed a policy of neutrality and non-alignment with Nato, but this sentiment changed after the Russian invasion of Ukraine in February 2022.
Although Nato was formed in response to the developing Cold War, it has since remained a key part of international defence and security, and remains the largest peacetime military alliance in the world.
More recently, Nato has co-ordinated a response across the alliance to Russia's invasion of Ukraine.
Whilst Ukraine is not in Nato, defence and security links between Kyiv and the alliance started soon after Ukraine's independence in 1991.
While the alliance supported Ukraine when Russia annexed Crimea in 2014, the member countries increased their support after Russia's invasion in 2022.
This has seen the US, Nato's biggest military spenders, stay true to form and commit the largest amount of foreign aid to Ukraine – $33.2bn.
The UK is the second largest donor to Ukraine, sending £2.3bn in military assistance in 2022 and pledging to match that number in 2023.
As well as donating weapons, Russia's invasion of Ukraine has seen Nato allies, such as Germany, change their stance on their defence policy – with Berlin sending Leopard tanks to Ukraine.
How does Nato operate?
All members agree to mutual defence in response to an enemy attack, under Article 5 of the Washington Treaty.
"An attack against one ally is considered as an attack against all allies," as the principle goes.
The first time and only time this was invoked was after the 9/11 attacks on the US.
Nato does not have its own armed forces, but rather coordinates each member nation contributing their own forces, which together are hoped to add up to more than the sum of their parts.
In total, Nato can count on more than 3.5 million personnel from the militaries across the alliance.
After the fall of the Berlin Wall in 1989 and the subsequent end of the Cold War after the fall of the Soviet Union in 1991, the purpose of Nato was called into question.
But the new era ushered in new threats, and Nato's original remit has now expanded.
These include instability on Nato's eastern and southern borders, terrorism, the proliferation of weapons of mass destruction (WMD), space, cyber attacks, threats to energy supplies and the security threats posed by global warming.
But its main principle has remained the collective defence of its members.
There are currently 20,000 military personnel engaged in Nato operations and missions around the globe.
Nato withdrew troops from Afghanistan in 2021, but still conducts operations in Iraq, Kosovo and the Mediterranean, plus deployments in the Baltics.
This includes Nato's enhanced Forward Presence and air policing missions on the alliance's eastern flank, brought into focus further by Russia's invasion of Ukraine.
Nato forces have previously fought on combat operations in places including Bosnia and Herzegovina and Afghanistan.
According to recent Nato estimates, the US leads the way in terms of troop numbers, followed by Turkey, France, Germany and Italy.
How is Nato funded?
Nato sets its members a target of spending at least 2% of their GDP on defence.
Previously, the failure of many nations to hit that target has led to some leaders questioning the alliance's effectiveness.
Throughout his first presidential campaign, former US President Donald Trump called the alliance "obsolete" and continued to criticise it during his time in the White House.
In the past, Mr Trump even threatened to pull the US – the alliance's biggest contributor by far – out of Nato.
France has also voiced concern over Nato's direction, with the country's president Emmanuel Macron saying the European Union must step up to avoid the alliance's "brain death".
However, new US President Joe Biden vowed to restore America's relationship with the alliance during his presidential campaign.
The Ministry of Defence said in 2022 Nato's spending has increased by $59bn in total since 2020 when adjusted for inflation, of which the USA accounted for nearly $23bn.
According to the latest details, seven Nato countries are meeting the guideline to spend 2% of GDP on defence.
These countries are Greece, the USA, the UK, Poland, Latvia, Estonia and Croatia.
The UK remains the second largest spender in Nato, after the US.
Nations, such as Germany and Canada, however, are currently failing to meet the guideline spend.
Which countries are in Nato?
- North Macedonia
- United Kingdom
- United States