Factbox: Blood and treasure or fantasy - What is the UK-U.S. 'special relationship'?

FILE PHOTO: U.S. President Donald Trump greets Britain's Prime Minister Theresa May during a bilateral meeting on the sidelines of the 73rd session of the United Nations General Assembly in New York, U.S., September 26, 2018. REUTERS/Carlos Barria

LONDON (Reuters) - Britain's so-called special relationship with the United States was one of the most enduring alliances of the 20th century, though Brexit and the presidency of Donald Trump have raised questions about its future.

Trump begins a three-day visit to Britain on Monday almost a year after he created controversy during his last trip by praising Prime Minister Theresa May's rivals and criticising the way she was handling the Brexit negotiations.