At the 2019 NATO summit in London, the Canadian prime minister, Justin Trudeau, had the misfortune to be caught on camera joking with other world leaders about Donald Trump. The US president was not amused, later calling Trudeau "two-faced".
Yet mockery is one of Trump's favourite political tools. In his tweets and campaign rallies, he often pokes fun at political rivals. And his mockery is not limited to critics in the United States. Trump also uses it against fellow world leaders.
Take Kim Jong-un, president of North Korea. In 2017 Trump mocked him as "little rocket man" and boasted that his nuclear button was "much bigger" and "more powerful" than Kim's. Not just a diplomatic put down, but a personal one.
The North Korean gave as good as he got, calling Trump a "dotard", or feeble old man. But rather than creating a diplomatic standoff, the exchange led to a summit between the two men in June 2018. It remains to be seen if Kim's recent description of Trump's presidency as "the dotage of a dotard" is an old joke told once too often.
At the NATO summit, the British prime minister, Boris Johnson, was one of the leaders filmed laughing along with Trudeau. Afterwards he claimed not to recall it. A shame, as he may have enjoyed the joke. In his former role as foreign secretary he was known for his use of wit.
"Telling jokes is a very effective way of getting a diplomatic message across", he told the House of Commons foreign affairs select committee in 2017. People like it, he maintained, "that you are talking to them in that informal way while subtly getting your point across".
At one diplomatic gathering, Johnson had declared: "We have invaded, defeated, or conquered most of your countries but we are here as friends". The audience of Arab and African ambassadors may not have been amused.
In October 2017, Johnson quipped that the city of Sirte in war-torn Libya could be a new Dubai. "The only thing they've got to do is clear the dead bodies," he said. On another occasion he warned an Italian minister that Prosecco sales would plummet if they didn't keep Britain's bubbly-swilling citizens in the European single market.
In Johnson's defence, humour in diplomacy is nothing new. Preparing to march his army into Sparta in 346 BC, Philip of Macedon is said to have asked the Spartan king if he should come as friend or foe. The reply was "neither". Threatening to burn the Spartan capital to the ground if he took it by force, he got another one word response: "If".
In Shakespeare's Henry V, set in the 15th century, the king receives a diplomatic envoy from France. He brings a gift from the dauphin, the heir to the French crown. Not jewels or fine wine, but a barrel of tennis balls, mocking the king's former playboy lifestyle. In reply, Henry compares the "Paris balls" to English cannonballs. Like Trump, he knew how to combine military threat with personal insult.
Indeed, gifts can be a good way to make a joke. In the 1770s, Louis XVI became irritated by the popularity of Benjamin Franklin at the French court. He reportedly showed his feelings by giving one of the American diplomat's female admirers a chamber pot with a picture of Franklin on it.
Who's laughing now?
The best humour can be unintended. At the United Nations General Assembly in September 2018, Trump said his administration "has accomplished more than almost any administration in the history of our country". A remark met by laughter, to the surprise of Trump, who admitted he "didn't expect that reaction".
At times it's hard to tell if the actions of a world leader are intended as humour or not. Take Nikita Khrushchev. Historian Martin McCauley states that the Soviet leader "shouted, laughed" and "banged his shoe on the desk" during a speech by then British prime minister, Harold Macmillan, at the United Nations General Assembly in October 1960.
One downside of a sense of humour is that it doesn't always go down well in other cultures. Even when they speak the same language.
In November 1940 Joseph Kennedy, then the American ambassador to the United Kingdom, gave an interview to the Boston Sunday Globe newspaper. Praising the Queen, he claimed "she had more brains than the cabinet" and that democracy in England was "finished". In this case the joke was on the ambassador, who resigned soon after.
More recently, in July 2019 Sir Kim Darroch, the British ambassador to Washington, was forced to resign after a series of his emails to the Foreign Office about the Trump administration were leaked.
The emails were mostly unflattering, but the president may have enjoyed being compared somewhat humorously to the Terminator character played by Arnold Schwarzenegger - an indestructible being who can walk unharmed through the fire and chaos he creates. It may even be an approach he takes to the next NATO summit.
Author: Kevern Verney - Associate Dean (Research) and Professor of History, Edge Hill University