UK Prime Minister Sir Keir Starmer says a “huge amount” has happened since his “coalition of the willing” idea first surfaced at his Lancaster House summit a fortnight ago.

He is not wrong: US-Ukrainian relations have been on a rollercoaster since then, culminating in the meeting in Riyadh earlier this week, where the two sides agreed on a 30-day ceasefire.

But Sir Keir’s coalition is a big, still somewhat nebulous undertaking, and there is clearly a great deal of work to be done before this ad hoc alliance is ready to take on something as complex – and potentially perilous – as keeping the peace in Ukraine.

Sir Keir says the coalition is now bigger and that “new commitments” are on the table, though he did not spell these out.

Participants of Saturday morning’s virtual summit, he said, had agreed to keep military aid flowing to Ukraine and tighten restrictions on the Russian economy, to weaken Russian President Vladimir Putin’s war machine.

Planning, he said, would now move to an “operational phase”, with military chiefs due to meet in the UK next Thursday.

“Overall, we are successfully gathering political and military momentum,” he said.

It is likely that we will see a rolling set of political, diplomatic and military gatherings as the plan slowly takes shape.

It is far from plain sailing.

Asked about vital US military support for any European-led operation – what’s being called a “backstop” – the prime minister was clear: the US position had not changed.

European national security advisors including Jonathan Powell – one of those credited with convincing Ukraine’s President Volodymyr Zelensky to accept the US ceasefire proposal – were in Washington on Friday.

Unless US President Donald Trump’s position on the backstop changes, Sir Keir’s coalition of the willing could struggle to get off the ground.

For Zelensky, the military clock is ticking, especially in Kursk, where his troops have been occupying a shrinking sliver of Russian territory since August 2024.

Ukraine vehemently denies reports that its forces are surrounded in Kursk – a theory promoted by Trump on Friday – but they are clearly under enormous pressure and are losing ground.

When I was in Kyiv towards the end of last year, Ukrainian troops told us they were holding onto territory in Kursk as a bargaining chip to be played in future negotiations.

But as those negotiations approach, it is a chip that Putin seems determined to remove from the table first.

That may go some way towards explaining his “yes, but” approach to the idea of a 30-day ceasefire.