Chris Mason: Joe Biden and Keir Starmer try to second guess Putin

In the hours before the prime minister was taken by motorcade to the White House, he and his team were in a secure room at the nearby British Embassy.

This is a room designed for conversations spies are not meant to hear, however sophisticated their techniques for eavesdropping and intercepting digital exchanges.

The Downing Street team were talking to British government staff in Ukraine and Russia, assembling their briefing and approach for their forthcoming conversation with President Biden.

They arrived at the White House in the late afternoon Washington time, the president showing Sir Keir Starmer around the Rose Garden before heading for the Blue Room.

On each side of a long rectangular table, the two delegations, the prime minister and president with seven colleagues each alongside them.

For just a few minutes, we reporters were invited in too.

Warm words from the leaders followed by loud questions and prompt ejection for the journalists.

What followed was about 90 minutes of conversation in private.

Ukraine dominated, but not to the exclusion of other issues – not least the Middle East, China and Iran.

Downing Street had sought in advance to portray this as an opportunity for a deeper conversation than the usual round of international summits often allow.

But why bother when President Biden is soon to be yesterday’s man, out of office, power and influence in four months time?

The urgency of the issues on the table, I am told.

Take Ukraine: an ally of both the UK and the US, still in desperate need of ongoing help as its friends weigh up how best to provide it – and at what cost.

The UK has been “forward facing” as it was put to me in making the case to others to agree to Kyiv’s request to be allowed to fire western missiles into Russia.

President Biden is sceptical, fearful it could drag America and Europe into direct conflict with Moscow.

That is just what Vladimir Putin has been hinting at in the last few days.

Then again his sabre rattling in the past hasn’t come to much, so perhaps it wouldn’t again?

But maybe, this time, it would.

Diplomacy and intelligence turning to the psychology of a leader at war, attempting to second guess how he might react.

Would he really contemplate a military attack on a Nato member state – with the frightening potential of hauling the whole western alliance into war with Russia?

And, if not that, would Ukraine’s allies stomach lower level aggression in retaliation, such as cyber attacks or damaging sub-sea communication cables?

There was little expectation this meeting would resolve the question about western missiles, not least because further conversations with others at the United Nations are expected shortly.

Afterwards, the prime minister wouldn’t be drawn on whether he had persuaded the president to change his mind.

This is a conflict without obvious end which presents too no end of thorny dilemmas based around a recurring theme: how to defeat Russia without provoking Moscow.

What could be the consequences of action?

And what could be the consequences of inaction?

It is the essence of the West’s challenge since the full scale invasion of Ukraine two and a half years ago.