“Relieved is an understatement”.

That’s how one senior government figure described their mood as the prime minister, his foreign secretary and the Number 10 team left the White House having cemented Sir Keir and President Trump’s personal relationship; secured a commitment to NATO; the prospect of a trade deal that could spare the UK from tariffs and the presidential nod on the Chagos deal.

That gives you a sense of just how much was at stake going into talks with a president that has heaped criticism on many old allies in recent weeks.

That the Starmer-Trump meeting went as well as it did is proof, say those around the prime minister, of how much legwork and meticulous planning has gone into pulling it off.

The work began in opposition with Keir Starmer calling President Trump after the assassination attempt, while his foreign secretary courted vice president JD Vance.

There was the dinner in Trump Tower in September and, ever since then a prime minister who has swerved any personal or public criticism of the president.

When the visit was confirmed, the Number 10 team war-gamed all the possibilities with the prime minister, I’m told, going through all the detail and planning with aides to “make the most of the visit”.

They came out feeling that all the pieces fell into place with wins for the UK on trade and a “special” Keir Starmer coming through talks with President Trump that you could have a positive relationship with President Trump.

There were a few factors that helped the prime minister.

The first was the president’s genuine love for the UK and Sir Keir’s team’s understanding that President Trump is driven by personal relationships.

Get that right and much of the rest falls into place, say those who have seen him up close.

Sir Keir Starmer, the ponderous lawyer, has I’m told also built a genuinely warm relationship with President Trump: “They genuinely like each other and find each other easy to speak to,” is how one observer put it.

Then came the deployment of soft power.

I was in the Oval Office when the prime minister procured a personal letter from his jacket pocket from the King inviting President Trump for a second state visit.

Mr Trump was genuinely delighted as he read the letter, marvelled at King Charles’s signature and pointed out that this was the first time in history that anyone had been afforded such an “incredible” honour.

The prime minister also brought something to the table for a president that loves a deal.

The decision to lift defence spending to 2.5% of GDP by 2027 went down extremely well on Capitol Hill with Republicans and signalled to President Trump that the UK was an ally that had heeded his message on defence spending.

There were wins too.

Keir Starmer’s deal to transfer sovereignty over the Chagos Islands, where the US has a critical military base, to Mauritius, was backed by President Trump, a hugely welcome moment for a PM who has been heavily criticised about the deal back home.

And when I asked the president about his sharp remarks on EU tariffs – he said the union had been constructed to “screw” the US on trade – he told me he’d been having problems with the EU bit was in a “very different place” with the UK.

“I have a great warm spot for your country,” President Trump told me before the talks.

Afterwards, he told reporters at the press conference in the East Wing of the White House that there was a “very good chance” of a trade deal in which tariffs were not necessary.

Of course, there is a long way to go, but against a backdrop of a president threatening tariffs on a number of countries, that was a big win for the PM.

But if going out of this short visit was a story of trade, going into it was a focus on security guarantees for Ukraine.

Sir Keir said securing a backstop for European peacekeepers in Ukraine is the only way to contain President Putin.

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On this, President Trump was rather more noncommittal, stressing that the priority was to do the deal first and that the mineral agreement he is poised to sign with President Zelenskyy on Friday should be deterrent enough.

On Sunday, the PM will host 18 leaders in London to discuss what to do next.

Ultimately, Europe will need to know that the US has its back on Ukraine – the PM didn’t come away from the Trump meeting assured of that yet.

Although, the message for more NATO investment was heard loud and clear again, with an expectation from the British camp that more commitment on defence spending from Europe will be required by President Trump as part of any backstop deal.

What is clear coming out of this meeting is that Sir Keir and the UK could genuinely become the bridge between the US and Europe as the deteriorating relationship between Brussels and Washington offers Sir Keir the opportunity to take the centre stage.

After a bumpy start to his premiership, the prime minister has found in his endeavour on Ukraine, a means to show leadership – and perhaps a way to win doubtful voters back around.

He has managed to find a way with President Trump. Sunday’s summit is his next test as he brings together President Zelenskyy with fellow European leaders to try to hammer out a plan to support any peace deal.

Starmer was an unlikely war footing leader.

But he is finding a way.