Lancaster House may have been bathed in sunshine as European leaders gathered to chart a way forward for Ukraine.

But there was a cloud hanging over the summit amid the gilt-edged grandeur of the former royal townhouse in central London.

It came in the form of a question – was it all worth it? Was this gathering of European powers enough to shift the diplomatic dial? Or had the United States made up its mind to go it alone on finding a ceasefire in Ukraine?

The prime minister’s answer was very much that Europe still had a role to play. He said the UK, France and others would work with Ukraine on a plan to stop the fighting and then discuss that plan with the US.

He said Europe would keep military aid flowing now and, in the event of a peace deal, it would boost Ukraine’s own defensive capabilities, building up the country’s “armed forces and border defences” to ensure that “Ukraine can draw on munitions, finance and equipment to defend itself,” according to the official conclusions of the summit.

And Sir Keir said there would be a “coalition of the willing” to defend Ukraine and guarantee peace.

Who or what that would involve is not yet clear. The prime minister merely said that there would be “a force consisting of European and other partners”.

The UK and France have already said they would be willing to put boots on the ground in Ukraine to help deter Russian aggression.

The European Commission chief, Ursula von der Leyen, said the purpose was to turn Ukraine into a “steel porcupine that is indigestible for potential invaders”.

The aim, it would appear, is to beef up the European offer for post-war security guarantees for Ukraine in an attempt to convince Donald Trump to promise a US military backstop.

The president has thus far been reluctant to go down that route, suggesting the presence of US mining firms on the ground in Ukraine would be enough to keep the peace.

Sir Keir does not share this view, saying explicitly “to support peace on our continent and to succeed, this effort must have strong US backing”.

This summit was in many ways an attempt by Europe to take some ownership of the diplomacy. In recent weeks, efforts to end fighting in Ukraine have been led by Trump speaking to Russian’s President Vladimir Putin and at his Ukrainian counterpart Volodymyr Zelensky.

The Europeans also wanted to find ways of repairing the transatlantic alliance, which is looking pretty fragile after the confrontation between Trump and Zelensky in the Oval Office on Friday.

In recent weeks, the US administration has gone after its European allies; questioning their democracy, voting against them at the United Nations and – in the form of the president’s chief enforcer, Elon Musk – saying even the US should leave the Nato military alliance.

The summit in London was an attempt by the prime minister both to calm diplomatic brows and stiffen diplomatic sinews. Diplomats said there was clear resolve around the table to support Ukraine but also to build on Trump’s desire to find a ceasefire.

But Europe stepping up is only half of the story and much depends on whether the US is willing to play its part.

Diplomats say the US is willing to de-escalate the tensions with Zelensky, but there were mixed messages on Sunday. The US Secretary of State Marco Rubio said America was ready to engage with Ukraine, but the National Security Adviser Mike Waltz suggested Zelensky might have to resign as part of any ceasefire.

So the outcome of the Lancaster House summit was seemingly a united European front in backing Zelensky, with the promise of more military support for Ukraine to come.

But the cloud lurking in the London skies remains what the United States does next. And that is not yet clear. Trump and his deputy have made their antipathy towards Zelensky as clear as their trust in Putin.