The North Atlantic Treaty Organisation, commonly known as NATO, is turning 70.
Founded on 4 April 1949, with its first headquarters in London, the alliance has grown from 12 to 29 countries.
The Secretary General marked the occasion with a rare address to the US Congress.
Jens Stoltenberg acknowledged serious divisions within the alliance and called for bigger defence budgets to cope with global challenges, such as Russian assertiveness.
In the first address to Congress by a NATO head, he said:
"Questions are being asked on both sides of the Atlantic about the strength of our partnership. And, yes, there are differences."
The NATO Secretary General credited President Donald Trump with compelling allies to spend more on defence.
But he did not mention that President Trump has also questioned the value of the alliance and suggested that some members are freeloaders.
"Our alliance has not lasted for 70 years out of a sense of nostalgia or of sentiment," he said.
"NATO lasts because it is in the national interest of each and every one of our countries."
Mr Stoltenberg will lead a meeting of NATO foreign ministers, hosted by Secretary of State Mike Pompeo, to mark 70 years since the alliance's founding charter, the North Atlantic Treaty.
Imjin Barracks, in Gloucestershire, are one of Britain's leading contributions to the alliance as they are home to the Allied Rapid Reaction Corps, known as 'ARRC'.
On NATO's anniversary, 23 of the alliance's 29 nations whose militaries co-exist in Gloucester were determined to celebrate despite the weather.
"I'm glad that my country decided to fully join NATO again a few years ago," said Lieutenant Colonel Ben Rossillon from the French Army.
"We kept the values that were drafted 70 years ago in Washington," said Major General Maurizio Boni from the Italian Army.
"The alliance is an enduring alliance [with] enduring values."
Watch: celebrations took place in Imjin Barracks, Gloucestershire, home of the Allied Rapid Reaction Corps.
It is not clear what challenges face the alliance over the next 70 years, but the ARRC’s new enhanced role in NATO places the UK at the forefront of the alliance’s plans for how it will respond to whatever lies ahead.