Defence Secretary Sir Michael Fallon has promised that the United States will remain Britain's closest security partner after Donald Trump's inauguration as president.
General James "Mad Dog" Mattis has now been confirmed by the Senate as Mr Trump's defence secretary and Sir Michael drew attention to his experience as a former Nato commander as he congratulated him on taking up his post.
It comes after Mr Trump again raised concerns about his commitment to Nato at a time when Russia is showing increasing aggression.
In his inauguration speech on Friday, the new president complained that the US had "subsidised the armies of other countries" and "defended other nations' borders while refusing to defend our own".
Baltic nations, which fear Russian incursion as Vladimir Putin masses troops on their borders, will hope Mr Trump was simply urging other Nato members to boost their defence spending rather than signalling a desire to leave the alliance of mutual protection.
Sir Michael and Gen Mattis will be speaking at the earliest possible opportunity and have already exchanged welcome letters.
They are both are due to attend a meeting of Nato defence ministers in Brussels in February.
Sir Michael said: "I congratulate James Mattis on his confirmation as US Defence Secretary.
"With his experience as a former Nato commander, having also served alongside British forces, I look forward to working with him to build on our deep and enduring defence partnership.
'The US is and will remain our closest defence and security partner.
"Secretary Mattis and I will be working hard to ensure that both our countries remain equipped and ready to play our part in confronting the threats we face and deterring aggression."