I’ve been on quite a few trips now to the White House with successive prime ministers, but I can’t remember one that mattered as much as this. 

As Keir Starmer himself puts it, “everything has changed” and the prime minister finds himself negotiating with an old ally that is looking at the post-war world order afresh and doesn’t much like what it sees.

As Donald Trump appears to abandon the role the US took in the world during the Second World War and is now looking away from Europe and Ukraine, Starmer’s task is to try to win his attention, and support, once more.

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The prime minister’s two goals when he meets President Trump on Thursday are to help persuade him to put in place US security guarantees for Ukraine to deter Russian aggression post any peace deal, and dissuade President Trump from imposing tariffs on the UK.

That’s why, when it comes to UK security and economy – the two principal foundations of the Starmer administration – this meeting, this relationship, really matters.

It might be too much to say President Trump has the power to derail the Starmer project, but he does have in his gift the power to make Starmer’s twin task of guaranteeing British security and delivering economic growth far more challenging.

So the stakes are high for Britain and on the plane over to Washington that jeopardy was obvious to see.

When the prime minister came down to the back of the plane to answer questions from journalists travelling with him, he began the session with us by saying he had a lot of important things to discuss and was going to keep his answers short.

He then assiduously stuck to an agreed script, weighing each answer carefully as he swerved a number of questions on Ukraine and trade by saying he “wasn’t going to get ahead of discussions”, as he praised President Trump, referenced the special relationship as often as he could and insisted again the UK would not pick between the US and Europe.

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From the PM’s trip to Ukraine in January when he all but confirmed to Sky News that he was prepared to put British boots on the ground in a peacekeeping force, to his language around Europe stepping up at NATO in February, the European summit last week in Paris and then this week’s big commitment to accelerate the UK’s defence spending, the Number 10 team have been building up to this meeting, step by step.

But they are also all too aware that a positive outcome isn’t guaranteed when it comes to President Trump – and much will rest on what they find when Keir Starmer and Donald Trump meet in the room.

“We honestly don’t know what’s in his mind,” said one senior figure who has been war gaming the trip.

There were already bumps before the prime minister even landed. As Keir Starmer told reporters on the plane over that the security guarantees around any Ukraine peace deal “had to be sufficient to deter Putin from coming again” (and that requires a US military backstop in the minds of the Europeans), President Trump told reporters he was “not going to make security guarantees beyond very much”. “We’re gonna have Europe do that…Europe is their next door neighbour.”

Those remarks are perhaps not the ones the Number 10 team were hoping to hear, but they have a rule now to focus not on what Trump says, but what he actually does.

Much of what happens in the next 24 hours will be beyond the control of Sir Keir Starmer, but he has at least prepared the ground as best he can with a president who hates criticism, loves flattery and is very transactional.

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Starmer goes to Washington full of praise of President Trump – a leader he said on the plane over that he trusts – and a concrete pledge to put British troops into a peacekeeping force and lift UK defence spending, a key Trump ask of all NATO allies.

He has done the groundwork, now it comes down to the art of the deal.