NATO leaders are preparing to meet in the UK this week.
They will gather for a dinner at Buckingham Palace on Tueaday evening, before holding a day of talks in Watford on Wednesday.
Earlier this year, the alliance celebrated its 70th anniversary.
Formed in the aftermath of the Second World War, NATO's original goals were to secure peace in Europe, promote cooperation among its members and to counter the threat posed by the USSR, also known as the Soviet Union.
The alliance began with 12 member states but has since grown to 29, with former Soviet states also included.
NATO is set to welcome the Republic of North Macedonia as its 30th member, after an agreement was reached with Greece, who had blocked the membership.
Although NATO was formed in response to the developing Cold War, it has lasted beyond the end of that conflict.
It 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 co-ordinates each member nation contributing their own forces, which, taken 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.
NATO currently runs operations in Afghanistan, Iraq, Kosovo, the Mediterranean, as well as air policing missions in the Baltics and Iceland.
NATO forces have previously fought on combat operations in places including Bosnia and Herzegovina and Afghanistan.
How Is NATO Funded?
NATO sets its members a target of spending at least 2% of GDP on defence.
However, the failure of many nations to hit that target has led to some leaders questioning the alliance's effectiveness.
Throughout his presidential campaign, Donald Trump called the alliance "obselete" and has continued to criticise it ever since walking into the White House.
He has repeatedly urged other NATO countries to up their defence spending, claiming the US protects Europe and is then "unfairly clobbered on trade".
In the past, Mr Trump has even threatened to pull the US - the alliance's biggest contributor by far - out of NATO.
More recently, France has also voiced concern over NATO's direction, with Emmanuel Macron saying the European Union must step up to avoid the alliance's "brain death".
Out of the 29 nations, only nine currently meet the 2% spending target of which the UK is one.
Recent estimated figures show the US, Bulgaria, Greece, Estonia, Romania, Lithuania, Latvia and Poland also reach the target.
However, major nations including France, Germany and Canada are currently failing to meet the guideline spend.
NATO Secretary General Jens Stoltenberg argued the overall trend is positive with "five consecutive years of [spending] increase across Europe and Canada".