Chris Mason: The profound questions Zelensky’s European tour poses

Rishi Sunak (L) with Volodymyr Zelensky outside 10 Downing Street

Ukrainian President Volodymyr Zelensky's tour of Europe, for all its photo opportunities, gratitude and applause, is a reminder that the war leaves the UK and Ukraine's allies wrestling with profound questions.

It is a conflict without immediate, obvious end.

To how many requests from Ukraine can the answer be yes?

For how long?

At what cost?

And with what consequences?

To stand in Westminster Hall, with hundreds of others, was to see British politics come together.

Witnessing its invited speakers, first hand, is one of the greatest privileges of this job because people are invited to address the Hall at moments of huge import.

Last year, the King shortly after the death of Queen Elizabeth.

Some years ago - and before my time here - the Pope, Nelson Mandela and Charles de Gaulle.

And now Volodymyr Zelensky of Ukraine.

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This was a cross-party audience poised to applaud at any moment a leader who has become the face and voice of his country, and recognised in much of the world.

A defender of his nation and a defender of democracy.

But can the UK, should the UK, provide what he wants?

This was not just a lofty exposition of a speaker's worldview.

It was a leader at war with a specific request.

President Zelensky has regularly presented an ambitious list of requests to Ukraine's allies, many of which have often been met, even if not as quickly as he might have hoped.

And so to his latest: warplanes for his country's pilots to fly.

An observation and demand that amounted to "thanks for your help but we need lots more" was wrapped in the arresting rhetoric of a former actor, both comfortable on the public stage and savvy at the stagecraft of performance.

Presenting a fighter pilot's helmet to the Speaker of the Commons, Sir Lindsay Hoyle, he pointed to the writing upon it: "We have freedom, give us wings to protect it."

This was a whole lot more than Prime Minister Rishi Sunak is promising and is not easy to achieve.

And not only is President Zelensky asking for it, so is Boris Johnson, the prime minister-before-last who visited Kiev just last month.

So what should we read into the current prime minister's promise later that "nothing is off the table"?

In broad terms, we can read it literally.

The UK feels it has a strategic, diplomatic and moral obligation to remain steadfast in its support for Ukraine.

But Mr Sunak did not give a specific commitment to provide British planes.

There are several reasons for this.

Firstly, training pilots takes ages.

Secondly, fighter jets require complex engineering support to function properly in a warzone.

And thirdly, they would be vulnerable to Russian air defence systems, stationed within Russia - the targeting of which would risk a huge escalation in the war.

For all these reasons Lord Robertson, the former secretary general of Nato, told the BBC there were "huge problems" with the idea, at least in the short term.

But he did suggest that sending a dozen or so Challenger tanks, the first of which the prime minister told us will be on the battlefield under Ukrainian command next month, is not enough.

That, he said, is because "they are defending our front line" - the continent's front line, and the front line of our values.

Those words and this trip from President Zelensky are a reminder that the challenges and questions posed by this ongoing war to the UK and its European neighbours are likely to last for a long time to come.