Why Saudi Crown Prince Mohammed bin Salman's UK visit matters
Mohammed bin Salman is just 32 years old. He is Saudi Arabia's crown prince - not its head of state - and he has only been in his post for nine months.
Yet when this relative novice on the world stage arrives in London on his first global tour since taking office, he will be granted the reddest of red carpets.
There'll be lunch with the Queen at Buckingham Palace, and dinner with the Prince of Wales and Duke of Cambridge at Clarence House. There will be meetings with the prime minister at Chequers and Downing Street. And garland upon rhetorical garland of praise will be placed at his feet.
MBS - as he is known - will be granted this warm welcome not just because he is the de facto leader of his country. But also because Saudi Arabia has had a long alliance with the UK, a relationship that will take on a different shape after Brexit.
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For now, the relationship is based on deep and close security links. Saudi Arabia shares intelligence which the prime minister says has saved British lives on British streets.
The Saudis are keen on UK cyber expertise to help them tackle the threat from Iran. There is also a close defence relationship with Britain selling - controversially - billions of pounds worth of arms to Saudi Arabia, on which ministers insist tens of thousands of British jobs depend.
But this visit will not just be a case of two allies refreshing their relationship.
The Crown Prince is looking for international support for his internal economic reforms while at the same time trying to offer reassurance to nervous international investors. And the British government is keen to transform a security and defence relationship into one that includes broader economic ties as well.
The UK certainly supports the reforms that this young leader is introducing at an astonishingly fast rate. He is liberalising what has been for many years a deeply conservative country.
Women are being allowed to drive and go to football matches. Cinemas are being opened. There has been a crackdown on corruption with senior figures detained and forced to put billions back into state coffers.
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The Crown Prince has pushed back at the clerics who have held such sway for so many years, making the case for a more liberal vision of Islam. He has begun a process - known as the 2030 Vision - to try to make his country's economy less dependent on oil and more broad-based and market-orientated.
All this the British government supports, looking for opportunities for British businesses in the provision of education, entertainment, tourism and healthcare - all areas where officials believe the UK has expertise and a comparative advantage. The UK also has an unashamed appetite for inward investment from Saudi Arabia.
In particular, the British are keen to persuade the Saudis to float part of their massive state oil group, Aramco, on the London Stock Exchange.
This would be a hugely lucrative deal that has not surprisingly also attracted the attention of other global financial centres.
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Some MPs worry that the UK may water down its corporate governance rules to secure the listing. UK officials do not expect any early decision on this, but they will use the visit to make the case for London.
In other words, post-Brexit Britain will need allies, markets and money - and the Saudis are high on the UK's wish list.
But amid the diplomatic love that will be showered upon the Crown Prince, there will also be an appetite for reassurance. There are nerves about what some see as a young man in a hurry.
Some are concerned about reforms being forced on a conservative society so quickly that there could be a backlash at home.
Others worry about an anti-corruption drive that has made international investors nervous. Cracking down on graft is one thing, arbitrary detention of businessmen and confiscation of assets is another. The Saudis are keen, analysts say, to show that they are committed to a level regulatory playing field.
British diplomats have also been concerned by what some see as Mohammed bin Salman's impulsive foreign policy, in particular his feud with the Lebanese prime minister, Saad Hariri, who recently withdrew his resignation, a month after saying he was quitting in a shock announcement in Saudi Arabia.
Pushing back against Iranian influence in the region may be understandable, but not if it provokes yet more instability. The fear, say analysts, is that the Crown Prince is good at starting things, but less good at de-escalating them and managing risk.
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And then, of course, there are the issues on which the two countries have substantial differences.
The British government supports in principle the Saudi-led military campaign to restore the internationally-recognised government in Yemen, and accepts what it sees as Riyadh's legitimate security concerns.
But the UK has worries about the way the Saudis are prosecuting the campaign, particularly the excessive use of force and the thousands upon thousands of civilian deaths.
Thus far, all attempts to bring an end to what has become a proxy conflict with Iran have failed.
The Saudis continue to blame the continuing conflict on the Iranian-backed Houthi rebels, but privately the UK is still looking to play a role in finding some kind of political solution.
Britain is also keen to see an end to the standoff between Qatar and Saudi Arabia and its Gulf allies. For now, the Saudis appear happy with a status quo that they believe keeps Qatar on the back foot.
The UK is also at odds with the Saudis over the Iran nuclear deal, which Riyadh opposes. If the Saudis push back against Iran too firmly, there are fears that the nuclear deal could be endangered.
And then there is Saudi Arabia's human rights record.
Campaigners are concerned about what they see as Saudi Arabia's repressive actions, such as the detention of political activists and its widespread use of corporal and capital punishment. For all the Crown Prince's liberalisations, Saudi Arabia remains an autocratic monarchy which opposes dissent.
Theresa May says she will talk "frankly and constructively" about what she calls "issues of concern including regional security and the conflict and humanitarian situation in Yemen".
But that will not stop substantial demonstrations taking place throughout this visit. Many campaign groups are planning protests outside Downing Street.
Politicians are expected to join them. Their focal point will be the £4.6bn worth of aircraft, helicopters, drones, grenades, bombs and missiles which the Campaign Against the Arms Trade claims Britain has sold to Saudi Arabia since its bombardment of Yemen began in 2015.
How the British government and the Saudi prince respond to this will be a key test of this visit. The hope within the British government is that this will not sour an already sensitive visit on which much is at stake.
The Crown Prince wants to reassure global allies about his reforms and project a little Saudi economic leadership. And Britain wants to broaden its economic base after Brexit, and show its allies that it can remain a reliable partner in business and diplomacy.