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A meltdown. A horror show. A pile-on. However you want to describe it, the meeting between US President Donald Trump and President Volodymyr Zelensky in the Oval Office last night was a catastrophe: Ukraine and its most important ally in a public slanging match.

But more important than the insults and shouting was the clash on substance. America's threat to cut its support for Ukraine if Zelensky doesn't sign a deal. Trump's refusal to name Russia as the clear aggressor and Ukraine as the clear victim in the conflict.

It was a total contrast to what happened on Thursday when Prime Minister Sir Keir Starmer met Trump. "Ecstatic", "triumph", "outstanding" were a few of the words used by the PM's team.

They weren't just delighted with the optics of the two leaders chatting about their families and the royals over winter greens and sea bass – Sir Keir's team believed they had made genuine progress in talks in the Oval Office about Ukraine.

Trump told Sir Keir that the US was working fast on a ceasefire deal, but it wasn't done and they weren't rushing to a bad one. And the PM got an assurance the US was looking at how it could guarantee Ukraine's security if a peace was agreed. A senior government source told me "there are good reasons to be optimistic" about Trump's true position.

But last night's outbursts give a very different picture, and this morning you'd be hard pushed to find an optimist in Westminster.

A government minister here said the scenes had been "deeply troubling and sobering". Sir Keir spoke to both Trump and Zelensky last night, but No 10 is tight-lipped on what was said.

Today the Prime Minister will be in "back to back calls" trying to patch things up and this afternoon he is hosting Zelensky, who landed in the UK this morning, in Downing Street, ahead of a summit of European leaders on Sunday.

But Trump's words from last night suggest that both Sir Keir and French President Emmanuel Macron had not succeeded in one of their primary objectives this week: to persuade America that Russia is the aggressor and Ukraine must be protected.

So what now? Sir Keir has some difficult questions to ponder. Does he strive to get his new diplomatic friend at the White House back on board? Or is the new reality that the US just can't be relied on, that the animosity between Trump and Zelensky runs too deep.

If America's out, does he push European allies to join the UK and France in offering troops that could guarantee a peace deal – the "reassurance force" plan we talked about last week.

Is it even feasible for Europe to support Ukraine on its own?

I'll be asking the prime minister these questions myself when he joins us live in the studio tomorrow morning at one of the most critical junctures for both his leadership and the security of our continent.

The irony is that until last night's drama, conventional wisdom in political and diplomatic circles was that slowly but surely, the direction of travel was towards a deal to end the war. Whatever you think of him, the election of Trump had changed the dynamic and focused minds on bringing an end to the conflict.

No western allies would suggest that Zelensky should just fold after three years of holding off Russia, with all the lives lost and all the suffering of his people. Yet privately, and tiptoeing towards it in public, there was a sense that he was willing to take part in some form of negotiation.

His previous stance of not giving an inch had visibly softened in recent months. If the conflict is to end, most diplomats would tell you that has to happen at the negotiating table. For all the hot tempers, Trump had said he could get Russia to talk. To stop the war, Zelensky may have to get back to the table too.

In the next crucial 24 hours, Sir Keir will hope to pick up the pieces of last night's disastrous encounter and push Europe to come up with the strongest expression of support, alongside potential promises of more military backing.

He'll also hope he can reset the tone and calm the atmosphere.

The version of Trump administration we saw in the Oval Office last night does not portray a reliable partner.

The prime minister is now faced with a decision not just on what to do, but who he can trust.

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