US Secretary of State Mike Pompeo visits London on Wednesday to salvage a post-Brexit alliance with an old friend whose defiance on China and Iran underscores Washington's diplomatic isolation.
Prime Minister Boris Johnson has been touting the prospects of a big new trade deal with the United States that can fill the void of Britain's departure from the European Union on Friday.
But Pompeo's meetings with Foreign Secretary Dominic Raab and Johnson on Wednesday and Thursday threatens to become a damage limitation exercise for the "special relationship".
Pompeo had been doing his best to convince Johnson that allowing China's Huawei tech giant to help build Britain's next-generation 5G network introduced a long-term security threat.
Britain approved a limited but still important role for Huawei on Tuesday.
"The United States is disappointed by the UK's decision," a senior administration official said in a terse statement.
And Johnson had been pushing Washington to send back the wife of a US diplomat who is using the cover of diplomatic immunity to avoid prosecution over the death of a teenager in a road accident in England last August.
The United States rejected Anne Sacoolas's extradition to Britain last week.
"We feel this amounts to a denial of justice," Raab said in response. "The UK would have acted differently if this had been a UK diplomat serving in the US."
Disagreements about Iran and a US prosecutor's complaint that Prince Andrew was stonewalling an FBI investigation into the late sex offender Jeffrey Epstein only add to the layers of tensions.
Johnson might now simply be hoping "there are no precipitous statements from the US saying that's it, this love affair is at an end", said Ian Bond of London's Center for European Reform.
Bond pointed out that US officials had repeatedly warned how Huawei's inclusion in Britain's 5G rollout could force Washington to stop sharing intelligence with London.
"It would probably be safe to say that the British have not taken those (threats) very seriously and now is the moment we will find out whether they should have taken them more seriously or not," Bond told AFP.
Johnson may have got a sense of what comes next in a phone call with US President Donald Trump that followed his national security team's final decision on Huawei.
Downing Street said Johnson "underlined the importance of like-minded countries working together to diversify the market and break the dominance of a small number of companies".
Britain will cap Huawei's share of the 5G market to about a third in a bid to create space for new companies to step in.
But Johnson's decision still tests the limits of his well-publicised friendship with the mercurial White House chief.
Trump has been a big fan of both Johnson and Brexit. He dangled the promise of a new trade deal with Britain by as early as the end of the year.
Those talks can formally get underway once Brexit takes effect on Friday at 2300 GMT.
Any comprehensive agreement would need ratification in Congress and none is expected when most Americans' attention is focused on the heated presidential election campaign.
But a bare-bones deal that lays the foundation for future talks could work in both Trump and Johnson's political favour.
Some cyber security experts said Johnson's decision to keep Huawei away from the "core" elements of the 5G network should appease Washington's technical concerns.
"Essentially what they've done is limit Huawei from large parts of the network," said the Royal United Services Institute think-tank's cyber research chief James Sullivan.
"You would hope this would blow over in the next few weeks."