Trump visit: A barometer of political power
State visits are impressive, intensely choreographed and downright strange affairs.
They are often notable for what goes wrong - did a visiting president break protocol by addressing one of the royals incorrectly? Did a soldier with a bearskin hat faint in the heat while on parade? God forbid anyone used the wrong golden fork at the state dinner.
Whether or not in the next few days it is revealed that one of the Trump offspring took a selfie with the corgis, or the Duchess of Cambridge let Melania Trump try on her tiara in the Ladies, strip away all the excess and it's really about an expression of power.
In the next couple of days, we'll see an important marking of the passage of time since decisive days in World War Two.
There will be tributes to the bravery and sacrifice of the Allied forces, and restatements of the US and the UK's commitment to a relationship that is vital for both, and will endure.
But the political cast, as ever, has a great bearing on how well the relationship between the UK and the US can work.
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Joint appearances by Theresa May and Donald Trump have been outwardly at least, extremely awkward. That's in part because he has the habit of giving his unvarnished view of her government before he touches down, splashing controversy around in the way he so clearly enjoys.
It's also been because the contrast between them is just so profound. He, the tycoon who seems to adore breaking the political rules, who vaulted his way to the Oval Office taking the US establishment by surprise.
She, the careful politician who gradually inched her way upwards through the machine of the political party she loves and hoped to protect. Mr Trump, who relishes baiting those who disagree with him, and taunting the media. Mrs May, who gives the impression she would rather be left alone with her red boxes.
This time that difference is all the greater because the prime minister is on her way out of the door, while the president seeks another term in office.
They will have some discussions on Tuesday certainly. No 10 is expected to urge the White House to take climate change more seriously, and to think carefully about its approach to Iran.
In the other direction, expect the US to raise concerns over involving the Chinese telecoms firm Huawei in developing British infrastructure, and of course, the tentative conversations there have already been about trading after Brexit are likely to continue.
But don't expect dramatic joint announcements on Tuesday. If the political outcomes are a barometer of power, the truth is that Theresa May's is fading - with the US and Donald Trump having at least half an eye on who is coming next.