The symbolism was plain to see.

Five years on from Brexit, the British prime minister on Monday was brought back into the club for one night only, invited to an informal dinner with the EU’s 27 leaders to talk about resetting relations after a bumpy Brexit.

The invite was sent out weeks ago, with the intention on both sides to forge closer defence, security and trade ties.

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Britain is under new leadership with a Labour government that wanted no part of Brexit, while the European Union is perhaps now a bit more pragmatic as it seeks to renew a lapsed friendship with an old ally in a more hostile world.

The stage was set then for a rapprochement – instead the EU leaders spent the informal summit locked up in a room war gaming the threat of a US-EU trade war.

President Trump’s hostility to the EU was laid bare as he sounded off about the “atrocity” of the EU on trade and said tariffs were on their way, while with the UK, the US could probably sort it out.

The substance behind these positions is that the EU has a whopping trade deficit with the US – Trump hates that – while the UK does not.

But there is something else emerging here too: a Labour prime minister and Republican president that seem to have hit it off.

There was the dinner they had in September at Trump Towers in New York and the 45-minute phone call a week ago on Sunday which I’m told was light on policy and instead a very personal call between the two of them.

One insider told me they were “surprised” at how amenable Trump is towards Starmer. They may not be an obvious pairing for a bromance, but no matter – it all adds to the favourable backdrop of a president with links to Scotland, his fondness for the UK and love of the Royal Family.

But the big unknown is whether Starmer can remain close to both the EU and the US should relations between Washington and Brussels deteriorate. The prime minister was very clear on Monday that he’s unwilling to pick between the two sides.

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Some of it depends on how the EU handles this unpredictable president. For now, the bloc is treading carefully, with intense talks going on between the US and EU over trade – could the EU for example take more oil and gas from the US market to close that trade imbalance? – as individual European leaders hold the line.

In Brussels and London, operatives are watching how the threat of tariffs plays out in Mexico and Canada, where it seems like making concessions garners results.

As for London, there is quiet confidence that Trump won’t slap tariffs on the UK. “That’s not where he’s looking”, says one government insider.

But what Starmer can’t be sure of is whether the US president will also look the other way as the UK forges a closer trading relationship with China, while standing firm with Europe should relations between Washington and Brussels deteriorate.

Starmer and his team think it can manage relations with China, the US and the EU so that the UK comes out on top. But that could prove much easier said than done.

As Donald Trump kicks off his threatened trade war by slapping tariffs on both friends and foes alike, Number 10 is preparing for the moment he turns his attention to the UK.

The unpredictability of the returning president, emboldened by a second term, means the prime minister must plan for every possible scenario.

Under normal circumstances, the special relationship might be the basis for special treatment but the early signs suggest, maybe not.

It was never going to be an easy ride, with Sir Keir Starmer’s top team racking up years of insults against Trump when they were in opposition.

The bad feeling continued when Peter Mandelson was proposed as the UK’s new ambassador to the US – prompting speculation he might even be vetoed.

Amid all of this, the much-anticipated call between the two leaders seemed slow to take place, although it was cordial when POTUS finally picked up the phone last Sunday, with a trip to Washington to come “soon”.

It is against this slightly tense backdrop that the future of transatlantic trade will be decided, with Westminster braced for the impact of the president’s next move.

So, it’s unsurprising that as he waits, Sir Keir will spend the next few days resetting a different trading relationship – with Europe.

In this area, he is on slightly firmer ground, as the spectre of a global trade war makes European leaders want to huddle closer together to weather the storm.

And conversely, the Labour government’s track record works in their favour here, as they cash in their pro-EU credentials and wipe the slate clean after the bad-tempered Boris Johnson years.

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It is still, however, an ambitious and risky endeavour to begin the delicate process of removing some of the most obstructive post-Brexit bureaucracy.

For minimal economic benefits on both sides, the UK must convince the Europeans that they are not letting Britain “have its cake and eat it”.

At the same time, Brexiteers back at home will cry betrayal at any hint that the UK is sneaking back into the bloc via the back door.

To make it even trickier, it must all be done with one eye on Washington, because while a united Europe may be necessary in the Trump era, the prime minister will not want to seem like he is picking sides so early on.

As with so many things in politics, it’s a delicate balancing act with the most serious of consequences, for a prime minister who is still to prove himself.