It wasn’t yet official, but Sir Keir Starmer was straight out of the blocks on Wednesday morning to congratulate Donald Trump on his imminent victory – as America reeled from an election that turned from being too close to call into an emphatic, definitive and quick win.

The president-elect took the Electoral College, the popular vote, and the Senate.

The victory is hugely consequential, not just for a divided America that now has at its helm a president who ran a campaign that played on fear, economic and social insecurity and grievance, but for the UK and Europe too.

The US election has chosen a strongman leader for uncertain times and he enters the White House with a huge mandate. How he chooses to wield that power matters to us all.

For the Labour government, it will be a more trying diplomatic test than would have befallen a Conservative one.

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As Donald Trump noted when he and Sir Keir dined at Trump Tower in September ahead of the election, he and Starmer are not natural bedfellows.

“You’re a liberal so we won’t always agree,” he reportedly told the UK prime minister over dinner. “But we can work together.”

Sir Keir, a former human rights lawyer and a part of the sister party to the Democrats, and Donald Trump, a brash, deal-making billionaire businessman, are not an obvious match.

For Donald Trump, relationships matter. He hit it off with Boris Johnson but never really rubbed along with Theresa May.

Sir Keir’s approach will be to keep calm and carry on. I’m told the PM is of the view that “it is not about what Trump says but what he does”.

He is obviously not the preferred choice of the Labour leadership, but those in Number 10 are pragmatic and have been preparing for the outcome for some months.

“For us, there will always be areas of common interest than transcend party politics, as does the special relationship,” one senior figure said.

“Our approach is that it’s ultimately for the American people, and they have clearly chosen who they want to be president – and our responsibility is to make the relationship work in the UK national interest.

“That’s why the PM took time to have dinner with President Trump in September – and that was a very successful evening. It’s fortuitous to have that time so we are not starting from scratch.”

There have already been moments of possible tension that the prime minister has brushed off as inconsequential, aware that it is unwise to poke the bear.

When, on our way to the Commonwealth summit in Samoa, Trump’s election team filed a complaint against Labour, accusing it of “blatant foreign interference” in the US election in aid of the Harris campaign.

This came after media reports about contact between Number 10 operatives and the Harris team and apparent volunteering efforts.

Sir Keir calmly brushed aside concerns and refused to rise to the bait – despite some of his most senior staff being personally targeted in the Trump complaint.

Meanwhile, David Lammy, the foreign secretary, has spent a good portion of his time in recent months with Republicans – with his supporters insisting the ground work has “paid off”.

“They know him now and he has good relations with JD Vance,” one supporter said.

I’m also told that past sharp criticisms of President Trump by Mr Lammy – he once called Trump a “neo-Nazi sociopath” – are long forgotten.

“He won’t hold a grudge if you treat him with respect when he’s in office,” insisted one insider.

The coming months will see whether that proves true, but government insiders point out to me that the reason Sir Keir took Mr Lammy to the Trump dinner in New York was to test the water.

“It was a successful evening. If there had been an issue, President Trump would have said something,” one said.

Sir Keir ever pragmatic, the mood more widely in government was pretty flat on Wednesday, as London woke up to the Trump win.

Many Labour staffers had gone over to support the Harris campaign, other members of the Starmer team had attended the Democratic convention in a time-honoured tradition of these two sister parties.

This was a government that had wanted a Democrat win.

As one insider said to me on Wednesday morning: “I hoped right up to the last moment that he wouldn’t win. But he has and here we are.”

There is plenty of anxiety in the UK about what Trump 2.0 might mean.

For starters, he has threatened across-the-board trade tariffs on all trading partners of 10% to 20%, while floating special treatment for the US’s chief rival China, with tariffs of up to 60%.

Blanket tariffs would hit billions of pounds of UK automotive, pharmaceutical and liquor exports.

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One senior government figure told me they have been wargaming the scenarios but did not know how the coming months would play out, noting the political win for Trump at home could make him bolder still.

“We are very well prepared, but you have to accept unpredictability is a key feature of Trump,” the senior figure said.

“The UK does not have a trade deficit on goods with the US, so we might not be top of his hit list, but a clean sweep like this [politically] probably makes him feel he has been totally validated.”

Those scenarios range from full-blown trade wars to more mildly protectionist measures, which the UK has already lived with under the Biden Administration’s Inflation Reduction Act – which was designed to drive businesses to the US through hundreds of billions of tax incentives.

“What he says in the heat of the campaign and what actually happens might differ. We have a window of opportunity during the transition [Trump will be inaugurated on 20 January] to argue that tariffs will have a huge impact on the US too,” the senior figure said.

“It’s hard to say what it’s going to mean for us now. There will be trade implications, but it’s not clear whether they will be flat tariffs that spark a trade war with China in which we all feel the pain or mildly protectionist stuff, which the US has been doing for years.

“In the nightmare trade scenario of huge tariffs… if this happens, this is going to make Brexit feel like a papercut.”

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For the UK, outside of the EU and dealing with China and the US, tariffs could be particularly acute and cause difficulties domestically.

As a big trading nation, tariffs would have big inflationary effects on goods.

“It would have a big impact on how we have to face China and Europe and creates retaliatory trade wars,” one person familiar with the scenario planning said.

“It forces us to choose. Do we face more to America, do we face more to China?

“We are exposed as a trading nation as we are relatively little between the bloc of the EU, China and the US. So we will be disproportionately affected and we don’t have big friends to buddy up to, so we will be squeezed.”

It’s also not clear how flat tariffs tie into the much-lauded post-Brexit trade deal that Trump was once keen on but President Biden was not.

The Labour government for now are unclear how this might be revisited, or whether they want it to be, in the light of both Trump’s protectionism and the Labour red lines on opening up the NHS to private pharma or agriculture to imports of genetically modified products.

The prospect of punitive tariffs to force the UK to the table is the very opposite of what the government wants.

However, could there be some middle ground?

One government official argues there could be scope for cooperation on security or technology that is different to a full-fat trade deal.

There is some hope that the deal maker Trump might be open to such tie-ups.

For broader foreign policy, Trump in the White House is a significant change and the UK government is bracing for his different approach.

It is, insiders admit, hugely uncertain.

President Trump – in his focus in this election on the domestic US economy and borders – has not clearly spelt out how he intends to interact with the wider world on matters of Ukraine, the Middle East or China.

What is clear is that in Trump’s first term of power, he was often abrasive with old allies – he lambasted European friends for weak defence spending and even floated the prospect of leaving NATO.

Some NATO members now fear his return will lead to reduced US commitment to European security and an end to military support in Ukraine.

He has promised to end the war if he was returned to the White House, but has not spelt out exactly how he intends to do that.

This is true too of the Israel-Hamas war in Gaza and the Israel-Hezbollah war in Lebanon.

Trump has promised to bring “peace” to the Middle East, but has not said how. With his tough stance against Iran, Trump could end up offering more military US support for Israel. It is simply, for now, unclear.

What the UK government will now want to do in this transition time is to gain an audience with Trump and his team to press the UK and European interests.

However, there is an acknowledgement too within the government that it is now even more pressing for European allies to increase defence spending in the wake of a Trump win.

“Our approach will be to look at how he acts, rather than what he says,” one UK official said.

“He [Trump] has had an audience with Zelenskyy, who again has reiterated his ‘good discussions’ with Trump on the victory plan in September. But we agree with him, that Europe does need to do more [in spending].”

There is also a domestic question for Sir Keir in all of this.

Will the victory of Trump’s brand of populism and grievance politics fan those flames here in the UK?

Labour are all too alive to the threat of Reform, which won five seats, came second in dozens more and picked up 14% of the votes in the July election.

Sir Keir’s government knows it faces backlash should it fail to deliver on promises made.

There will also be obvious anxiety in government that Trump’s election in the US feeds into the global currents moving towards the populist right in Europe too, with Marine Le Pen now the bookmakers’ favourite to become the next president of France in 2027.

President Trump’s priorities will undoubtedly be domestic, as he told the American people in his victory speech he would keep the promises he made to seal the US-Mexican border, deport millions of undocumented immigrants and fix the US economy.

But his pledges to the American people touch on our lives too.

His promise of sweeping tariffs to restore the US’s manufacturing base could hit our economy, while his pledge to retreat from America’s role as global policeman to a more isolationist approach could affect Europe’s security too.

It’s the biggest comeback in political history – and it takes not just America into the unknown, but her old allies too.