US broke protocol with embarrassing flag mistake in front of King Charles

Diplomacy is all about protocol, pageantry, and presenting your national interests as best you can, but one small slip-up in this process can cause a major embarrassment.

This was on full display during King Charles III's final day in Washington DC, after the 77-year-old monarch and Queen Camilla had said goodbye to President Trump and First Lady Melania after a successful few days in the US capital.

On their way out, the British royals stopped by the Arlington National Cemetery to pay their respects to the fallen soldiers who had given their lives in two world wars to protect UK and American interests in Europe.

But the US military made a major mistake that would easily be spotted by any Brit worth their salt, with many spotting the issue with the flag as King Charles laid a wreath on the Tomb of the Unknown Soldier - representing all those whose remains lay in some corner of a foreign field.

King Charles' state visit has, mostly, gone off without a hitch (Andrew Harnik/Getty Images)

But while it is incredibly obvious which way up the red, white and blue of the Stars and Stripes is meant to be hung, the British Union Jack flag actually has a very subtle difference between its top and bottom half.

The thicker white diagonal lines closest to the flagpole are meant to be above the red diagonal line, even though the flag otherwise looks like you can fly it either way.

Britain's Flag Institute explains this difference as being tied to three countries of the United Kingdom - England, Scotland, and Northern Ireland - which are all represented in the flag. Wales was not included as it is technically a principality.

How the flag was flown vs how it should look (Heather Diehl/Getty Images/Getty Stock Images)

Just like the beloved flag of the United States, every part of the Union Jack's heraldry reflects the very fabric of the United Kingdom, with each color and stripe representing the 1000-year story of Britain and its formation.

The white 'Saltire' cross of once independent Scotland 'takes precedence' over the diagonal red cross used to represent Britain's centuries of domination over the island of Ireland, which nowadays is restricted to the six counties of Northern Ireland.

Which way up the flag is flown actually has a deep historical meaning for the British, whose Royal Navy ships once allowed them to rule the world's oceans. This is because, when you have hundreds of ships and no means of communication, the use of flags becomes very important.

Britain's navy would only ever fly their ensign, or flag, upside down if they were in some kind of distress. So flying the Union Jack the wrong way up is actually a major faux pas, just as it would be with the Stars and Stripes.

King Charles has finished his four-day state visit, which has been viewed as a success by all sides (Andrew Harnik/Getty Images)

"It is most improper to fly the flag upside down," the Flag Institute states.

And in a sign of how hard it can be to get all these diplomatic rules right, this was not even the only flag mishap that took place during the state visit by King Charles, who is also the ruler of Canada, Australia, New Zealand, and a number of island states around the world.

ABC reporter James Longman spotted the flag error, which happened earlier this week, on streetlights around the capital.

He shared: "For about two hours, they put the Australian flag up alongside the Stars and Stripes… I think they realized their mistake, and they've replaced them now with the Union Jack."

But if these mistakes had any impact on the king's diplomatic mission, it didn't show, with President Trump announcing a relaxation on whisky tariffs from the UK as a result of Charles' negotiations.