It was a small signifier – one which could be over-interpreted – but nevertheless provoked angst among some in government at a time when there isn’t universally huge confidence that UK-US political relations are as robust as they should be.

On Sunday, the British Embassy in Washington had a bash for members of the incoming Trump administration: a chance to boost goodwill amongst supporters of the 47th president as they prepared to descend on the nation’s capital.

Organisers hoped to get a very high-value guest indeed: someone who isn’t a household name, or a member of the incoming Trump cabinet, but who was set to become the most powerful attendee of the lot within hours – Susie Wiles, Donald Trump’s chief of staff and long-time political consultant.

Credited with injecting a level of professionalism in Trump 2.0 absent in 2016 and 2017, Wiles is so close to Trump that he invited her to take the microphone as part of his victory speech on election day in November. As she almost always does, she declined the limelight.

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There are many good and understandable reasons why Wiles would have been absent from a British embassy party hours before her man swore the oath of office and returned to the White House.

But her failure to attend was nevertheless a disappointment felt on both sides of the Atlantic. Just the latest source of insecurity, it seems, as the special relationship is put to the test. Insecurity at a very difficult moment.

As Donald 2.0 returns to the world stage, the rhetoric from Keir Starmer and David Lammy could not, superficially, appear more confident in US-UK links.

In remarks released on Sunday night, the PM talked about a shared history fighting common enemies and deepening the special relationship. But, behind the scenes, there is far less certainty that all is well – and we will soon see if this is true.

Nowhere will this be put to a faster, more brutal test than over the attitude of the White House, Trump allies and the president himself to Lord Peter Mandelson – the man Downing Street hopes will be the incoming British ambassador to Washington.

At some point in February, Lord Mandelson – known as the Prince of Darkness from his days as a New Labour spin doctor – will fly to Washington to present his credentials to the new president, a precondition of starting work in the job.

But, in truth, nobody really knows what will happen at that point: whether they will be accepted, and whether Lord Mandelson will be able to get on the job to which he has been tasked by Starmer.

In London on Sunday, government sources got to work claiming the suggestion his application to be the next inhabitant of 3100 Massachusetts Avenue could be rejected was just “bar room gossip”.

But it is not.

In parts of government it is being considered a clear and present danger. A full-scale diplomatic effort over the coming weeks will be put into securing Lord Mandelson’s position. The stakes are high, because of the way Number 10 has gone about this selection.

The fundamentals of our link to the US – the shared history, the Five Eyes security alliance, military and diplomatic cooperation – means a special relationship will almost certainly endure whatever the path of Trump’s second presidency.

But the political ties between the Labour Party and Trump-leaning Republicans are deeply frayed and recently got worse, according to multiple sources I’ve spoken to – and much of this could play itself out in how Trumpland chooses to treat Lord Mandelson in the next few weeks.

Some of those around Trump have made it clear that the incoming president is still minded to reject Mandelson’s credentials on two grounds: previous criticism of the president himself, and also over his support for China and business dealings with a country out of favour with the US.

All this against a backdrop of the first political diplomatic appointment to Washington in decades.

This means Lord Mandelson is also weighed down by all the baggage of months of rows over Labour’s efforts to campaign for rival Kamal Harris and the role of Starmer’s pollster Deborah Mattinson in the US during the election.

However, the manner of Downing Street’s appointment of Lord Mandelson was just as big a problem as the name itself, I’m told.

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Long before the election, Labour signalled a preference for a political appointee to succeed Karen Pierce, the respected incumbent.

But the months rolled on and Number 10 put back the decision, creating a vacuum which Ms Pierce filled by – what some saw as – a campaign to continue in post.

In Trumpland, she ensured she was regarded as the best woman for the job, and was – according to sources – even succeeding on being on the radar of President Trump himself.

The rest of Washington acknowledged her success getting close to the now president as well.

It took until December, and a visit by Starmer’s chief of staff Morgan McSweeney, before a decision was made that Lord Mandelson should get the job, and made clear he was starting in a matter of weeks.

It was an abrupt end to Ms Pierce’s time. Allies of the incoming president, who had an affection for Ms Pierce, were aghast. “Trumpland was horrified,” a source told me. “They were saying we want you to stay.”

Tales of their horror have ricocheted around Whitehall, with one source in the Foreign Office claiming they think Lord Mandelson is “dead on arrival” as it stands, and a huge effort is needed to reverse this.

Inside government, there’s an acknowledgement they did not initially fathom how grumpy Trump allies had become, and just how serious the consequences might be for a prime minister who has unexpectedly made foreign policy such a key part of the job.

There is now irritation in all direction. Whitehall sources said today Number 10 is cross with Ms Pierce, who they blame for making life more difficult for Lord Mandelson.

“They are briefing Karen is “vindictive” and better watch out or she won’t get an honour. Ridiculous. What 25 years of service and this is how it ends. This is disgusting – no respect,” they said.

Number 10 deny any rift, on the day Ms Pierce is the only representative of the UK government at the inauguration. The Foreign Office insist Ms Pierce has only ever been doing the job she was tasked to do.

Whatever happens, a big fight is on to ensure Lord Mandelson gets the post he’s been given.

Yet all this for a Labour figure that some claim is not as close to Sir Keir as some assume.

He is well-liked by the chief of staff, Mr McSweeney, but I’m told that Sir Keir has had flashes of irritation about Lord Mandelson both in the run-up to the general election and during the autumn – even as the selection process for ambassador was underway.

Ominously, there is precedent for Trumpland casting out a UK ambassador: Sir Kim Darroch was effectively excommunicated by Trump during the first presidency after disobliging cables written by Sir Kim were leaked while in office.

Lord Mandelson will make no such mistake – he wrote an ingratiating piece last Friday on the Fox News website praising Trump’s “straight-taking and deal-making”. He knows what he has to do.

The question now is how much Trumpland bears a grudge. The 47th president is at the peak of his power in the coming months, is signalling he is unwilling to back down, but it is unclear how many fights he wants to pick at once.

Sir Keir and David Lammy will both make trips to the US in the coming weeks, when ensuring the smooth transition of our man in the US is a priority.

But it is clear nothing is obvious. The failure of Susie Wiles to attend Sunday’s event prompted some to speculate maybe she was behind some of the briefings which ended up in the Sunday papers. With the fate of Lord Mandelson, as with everything else: nothing is certain in Trumpland – yet.