"Happy" was Keir Starmer's one-word verdict on his White House visit as he jetted out of Washington.

The Prime Minister grinned and gave a thumbs up to the travelling press pack as he boarded the flight home from the biggest test of his premiership so far.

Tensions had been high in the run-up to his meeting with Donald Trump after the President started Ukraine peace talks with Russian dictator Vladimir Putin - leaving European security hanging in the balance. But No10 aides could barely conceal their delight at how the talks turned out.

In an extraordinary 30-minutes in the Oval Office, Mr Trump heaped praise on the "special" PM and handed him diplomatic boosts on trade and the Chagos Island deal - one of the main attacks from Nigel Farage and the Tories.

The trip wasn't a total triumph. The US President again failed offer backing to European peacekeepers in Ukraine to deter Putin - Mr Starmer's top priority from the talks. But considering how unpredictable the President can be, the PM will have been delighted with the outcome.

The "Starm offensive" to win over Mr Trump has been months in the making. On the surface they seem like the most unlikely pairing.

Straight-laced Starmer, a stickler for the rules whose motivation is public service, and the wildcard US President, whose driving force is securing wins for his own ego. But Mr Starmer is also a pragmatist and knows it would be a long four years at war with the world's most powerful man.

British diplomats believe that Mr Trump is highly driven by personal relationships and that his often professed love of Britain is real, in part because his mother was born on the Scottish island of Lewis.

After the failed assassination attempt against Mr Trump last July, the PM swiftly spoke to him on the phone. And in September, they had a two-hour dinner at Trump Tower in New York, joined by Foreign Secretary David Lammy. Mr Lammy, who once called Trump a "neo-Nazi sociopath", recently let slip that he was personally offered seconds of the dinner by Mr Trump.

After Mr Trump's election victory, the strategy was to avoid getting drawn into slanging match with Mr Trump at all costs.

There have been moments when Mr Starmer has disagreed with the President - such as his claim President Zelensky was a "dictator" or the suggestion that Palestinians be expelled from Gaza to make way for a beach resort. But public rows have been kept to the absolute minimum.

Senior No10 officials were sent out to Washington last year and again in recent weeks to prepare the ground, while stateside staff have been on an intensive diplomatic blitz.

This week Mr Starmer sent a message to the White House that he was listening by announcing the biggest hike to UK defence spending since the Cold War. Increasing defence spend, which came at the expense of the overseas aid budget, has been one of Mr Trump's key asks of European leaders.

The night before his White House visit, Mr Starmer attended a glitzy do at the ambassador's residence to welcome New Labour stalwart Peter Mandelson to the job. Everyone was talking about the Trump meeting but officials were genuinely unsure about how things would go.

One UK source said after spending the day in Congress: "We're still liked here. But no one knows what's going on at the White House."

One the day itself, Mr Starmer was locked away for hours in briefings, taking on the advice of Mr Lammy and his closest aides. His tactic was to play his trump card first - a hand signed invitation for an unprecedented second state visit from the King.

Mr Trump - who adores the royal family - was clearly delighted by the offer, especially the fact that he's beaten his predecessors to get a second royal welcome. That broke the ice, and then the visibly more relaxed PM let the President take centre stage.

Over lunch of a winter green salad, grilled seabass and a chocolate caramel cake, the PM pressed him on Ukraine and trade, with Mr Trump later calling him a "very tough negotiator".

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The view on the British side is that the PM and the President genuinely seem to get on. Mr Trump appears to like the fact that Mr Starmer is straightforward and earnest, so he doesn’t feel like he’s being played.

He also rates Mr Starmer's overwhelming parliamentary majority - after winning his own decisive victory - and approves of the PM talking tough on the UK's interests.

Asked what they have in common, Mr Trump said: "He loves his country, and so do I." He added: "We like each other, frankly, and we like each other's country."

On the visit, Mr Starmer gave him a golf cap in his family tartan and a set of Downing Street golf balls in a nod to his love of the sport. A heavy dose of flattery, sticking up for Britain and a bit of royal glitz seems to be the winning formula for now.

But while this visit was a diplomatic success, Mr Trump could change his mind at any point. The UK is also not the only player here, with Russia's Vladimir Putin suddenly returning to the table. Behind Mr Trump also stand those like Elon Musk who are less enamoured with Britain.

And the resignation of international development minister Anneliese Dodds over the aid budget cuts hours after the PM returned to London shows that his decisions also have a cost at home.