US President Donald Trump headed for a historic second UK state visit hosted by his "friend" King Charles III Tuesday -- but political controversies, wars and tariffs threatened to cloud the pomp and pageantry.

Britain is rolling out its royal family in an extraordinary bid to woo the mercurial Trump, with security tight as protesters gear up to make their voices heard against the 79-year-old.

Trump, who has long been fascinated by the British monarchy, is the first US president to be invited for two state visits, after previously being hosted by Queen Elizabeth II during his last term in office in 2019.

King Charles will host Trump at Windsor Castle for a lavish banquet and carriage ride on Wednesday, before Trump meets Prime Minister Keir Starmer at his country residence on Thursday.

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