AP London

Windsor Castle staff are setting the 50-metre-long mahogany table. Grooms are buffing the hooves of the horses that will pull the royal carriages. And the military honour guard is drilling to ensure every step lands with precision.

Throughout the halls and grounds of the almost 1,000-year-old castle west of London, hundreds of people are working to make sure King Charles III puts on the best show possible when he welcomes US President Donald Trump for his historic second state visit this week. The visit, featuring glittering tiaras, brass bands and a sumptuous banquet served on 200-year-old silver, is a display of the pomp and ceremony that Britain does like no one else.

But it's a spectacle with a purpose: to bolster ties with one of the world's most powerful men at a time when his America First policies are roiling longstanding trade and security relationships.

"We're buttering up to him," said Robert Lacey, a royal historian and consultant on the Netflix series "The Crown." "He wouldn't come to Britain if he wouldn't have the chance to stay at Windsor Castle, probably pay homage to the (late) queen he admires so much, and to meet the king." 

 Soft power in action

Three centuries after Britain's kings and queens gave up political power and settled for the role of ceremonial head of state, the royals remain a robust instrument of "soft power," which the elected government uses to reward friends and wring concessions out of reluctant allies.

State visits are the monarchy's ultimate tool, with world leaders vying to get the full royal treatment.

During seven decades on the throne, the late Queen Elizabeth II hosted everyone from Romanian dictator Nicolae Ceausescu to South African President Nelson Mandela. The royals have also hosted the last four US presidents, though not all were full-scale state visits.

Hospitality with purpose

While the impact of soft power is hard to quantify, it contributes to a feeling of friendship that "may incline another party to be more open to your entreaties," said Martin Farr, an expert in modern British history at Newcastle University.

Six years ago, Britain sought Trump's support as it prepared to leave the European Union. This time, the UK is lobbying for favourable trade terms and help in combating Russian aggression in Ukraine.

"A new Trump presidency, a new prime minister, a different government, but the same sense of panic and the same feeling that the biggest lever we can pull with this president is to flatter him and to try and connect him with something he seems genuinely to be impressed by, which is monarchy, and the fact that his mother of course was born" in Scotland, Farr said.