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 cooperation among its members and to counter the threat posed by the Soviet Union.
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.
How Does NATO Operate?
All members agree to mutual defence in response to an enemy attack.
"An attack against one ally is considered as an attack against all allies," as the principle goes.
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.
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 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.
According to NATO estimates, only 11 member states, including the UK, contribute the desired 2% of annual GDP to defence.
Recent estimate figures showed that the US, Greece, Estonia, Poland, Latvia, Lithuania, Romania, France, Norway and the Slovak Republic contributed at least the 2% target.
Nations, such as Germany and Canada, however, are currently failing to meet the guideline spend.
Cover image: PA.