US Secretary of Defence General Jim Mattis has said America and Britain will stand shoulder to shoulder in facing down the increasing threat from North Korea and Russia.
Newly appointed Defence Secretary Gavin Williamson was hosting his US counterpart in London for a bilateral meeting following two days of Nato defence ministers' meetings in Brussels.
Congratulating him on his appointment, which came last week in the wake of sleaze allegations that led to the resignation of Mr Williamson's predecessor Sir Michael Fallon, Gen Mattis said it highlights the confidence Prime Minister Theresa May has placed in him.
In what they both called a volatile and dynamic security environment, Gen Mattis said they stand "united in condemning North Korea's aggressive provocation" and violation of international law, as well as the threat from Russia.
"Together we send a message of transatlantic resolve," he added.
"For many years Britain has galvanised the international community through the power of its moral voice, and today I look forward to strengthening that special relationship in defence of our way of life."
Describing British armed forces as a "national treasure", and one the US respects greatly, Gen Mattis thanked the UK for their efforts at the talks in Brussels to modernise Nato and an increased contribution to defeat Islamic State.
"The UK and the US maintain an unmatched, enduring relationship. It is not artificial or a historical artefact - it is in fact a pathway for the future," he added.
"It identifies common values and democratic ideals which I look forward to reinforcing alongside you."
Flanked by the Chief of the Defence Staff, Air Chief Marshal Sir Stuart Peach, Mr Williamson said the UK has been the United States' "closest partner for a century" - from fighting in the trenches of the First World War to tackling Islamic State.
"We face some major threats going forward, we have to be constantly vigilant to defeat the Daesh death cult. This year they have murdered citizens in New York, Manchester and London," Mr Williamson said.
"We must continue to confront increased Russian aggression, and deal with the disruptive and dangerous behaviour of North Korea. We must never, ever flinch in the face of aggression."
Highlighting how the UK is proud to be one of only six Nato countries hitting a 2% defence spending target, Mr Williamson also said they want to continue to build on the existing defence trading relationship - worth £3 billion a year.
"The United Kingdom and the United States, when we are united, quite simply, the world is a safer and better place, and let us make sure that unity continues going forward," he added.