Sir Keir Starmer faces the greatest dilemma of his life. When he stepped in front of reporters in Downing Street he looked like a man who has spent recent days staring into a terrifying abyss. How do you persuade Vladimir Putin – someone who has already sent thousands of his countrymen to awful deaths in an illegal war – to stop fighting?

He used his press conference to deliver a message straight to the Kremlin: “Let the guns fall silent. Let the barbaric attacks on Ukraine once and for all stop and agree to a ceasefire now.”

The Prime Minister had just finished a video call with world leaders. They need to decide how they will crank up the pressure on Putin so he comes to the negotiating table – and they also must agree how they will protect Ukraine so any ceasefire will not be the prelude to a further and even more destructive Russian invasion.

The UK lacks the military firepower of the United States but Sir Keir is attempting to lead by example, stressing the commitment that Britain is prepared to make to secure peace in Ukraine. He said the UK is willing to provide “troops on the ground and planes in the sky”.

Decisions taken in the coming days could shape the history of Europe for decades. Sir Keir must know this present conflict has the potential to spiral into an even more terrifying conflagration.

As he put it, “what happens in Europe always washes up on our shores”.

Military leaders will gather in the UK on Thursday to discuss the practicalities of making a peace deal a reality and then securing Ukraine’s future. The PM and his counterparts are also looking at new sanctions to push Russia towards a deal.

A key challenge for Sir Keir is keeping up a sense of momentum and strengthening belief that a just peace is possible. He was quick to stamp on any suggestion there is distance between the United Kingdom and the United States on Ukraine.

Britain and the US are talking every day, he said, adding: “I am clear in my own mind that President Trump is absolutely committed to the lasting peace that is needed in Ukraine and everything he is doing is geared towards that end. I am also absolutely clear that the relationship between the UK and the US is the closest of relationships between allied countries; it has been for decades and will continue to be for many years ahead.”

The PM also hit out at any suggestion Ukraine is culpable for the conflict, saying: “President Zelensky has shown once again and beyond any doubt that Ukraine is the party of peace.”

He believes Putin is trying to delay progress towards peace. Diplomatic endeavours are always endangered by inertia and ripe with potential for destructive divisions.

Sir Keir now finds himself trying to exert “maximum pressure” on Russia while binding together a “coalition of the willing”, with one of the most unpredictable US presidents in decades pursuing his own foreign policy from Washington DC. Sir Keir would not have entered politics if he did want the chance to make history but the weight on this barrister’s shoulders is immense.