Canada's Prime Minister Justin Trudeau, centre, gestures as he speaks during a NATO reception. Picture: Host Broadcaster via AP

It's rarely nice to hear others' unvarnished views about you. But when you're the president of the United States - even one whose craving for affirmation requires a daily avalanche of false and misleading statements - the candid truth shouldn't be such a shock.

On Tuesday, a group of world leaders gathered at a reception at Buckingham Palace. Unaware they were being filmed, they appeared to discuss the elephant in the room. "I just watched his team's jaws drop to the floor," Canadian Prime Minister Justin Trudeau said to the circle, which included French President Emmanuel Macron and British Prime Minister Boris Johnson.

His name was not mentioned in the clip, but President Donald Trump had little doubt he was the elephant. On Wednesday morning, he abruptly canceled a planned news conference to head home early, telling reporters that the Canadian prime minister was "two-faced" and adding that Trump had "called him out on" Canada's low defense spending.

Trump was later heard on an audio recording praising his own response. "That was funny when I said that guy was two-faced," he told an unidentified attendee of the NATO summit.

But by the standards of world leaders' insults, the chatter from Trudeau and his circle, a response to Trump's insistence on a marathon of news conferences on Tuesday, was mild. Foreign leaders are basically just ordinary people with bigger egos; they love to moan when they appear to be in private.

Often, their secrets come out only when they think the mics are off. Numerous world leaders have been caught giving unflattering remarks about their peers over the years. "What more does this housewife want from me? My balls on a plate?" French President Jacques Chirac said of British Prime Minister Margaret Thatcher during one memorable European meeting in 1988.

Another French leader, Nicolas Sarkozy, was speaking to President Barack Obama in 2011 when he offered an undiplomatic view of Israeli Prime Minister Benjamin Netanyahu that was accidentally broadcast to journalists and later published on French websites. "I cannot bear Netanyahu. He's a liar," Sarkozy said, before Obama lamented that he had to "deal with him even more often than you."

American leaders aren't always so contrite. As captured by the tapes he made of his presidency, Richard Nixon in 1971 called Canadian Prime Minister Pierre Trudeau, Justin's father, a "pompous egghead," accompanied by other, much saltier language. When the elder Trudeau found out he had been called one particular insult by Nixon, he responded: "I've been called worse things by better people."

Even the special relationship between Thatcher and President Ronald Reagan in the 1980s may not have been so cordial. Later accounts from British officials during the period suggest there were major policy differences between the pair, with Thatcher apparently worried about the U.S. administration's "amateurishness and disarray" in the Middle East.

"If I reported to you what Mrs. Thatcher really thought about President Reagan, it would damage Anglo-American relations," Nicholas Henderson, the British ambassador to Washington under Thatcher, later once told a British politician.

For some Trump supporters, the laughter behind his back was proof that Trump was doing something right. Laura Ingraham, a Fox News host, suggested that it was "great news" for Trump and said that all Republican presidents since Reagan had been mocked by foreign leaders.

Certainly, both Reagan and President George W. Bush were looked down upon by their foreign peers. Britain's Johnson, in a past life as a columnist and editor, once wrote that the younger Bush was "a cross-eyed Texan warmonger, unelected, inarticulate, who epitomizes the arrogance of American foreign policy."

But there's something different about Trump's relationship with other world leaders. He is much more pugilistic in his instincts, willing to engage in vicious spats with foreign officials ranging from North Korea's Kim Jong Un to London Mayor Sadiq Khan. And scores of foreign officials have been more than happy to insult him publicly and privately.

When push comes to shove, Trump can respond forcefully: He effectively forced a British ambassador out of Washington after the diplomat's private and unflattering thoughts on Trump were leaked. But at points, he has been blindsided by the mockery, such as when a remark during a United Nations speech that said his administration had "accomplished more than almost any administration in the history of our country" drew laughter.

"America's - so true," Trump said, stumbling over his words and visibly surprised. "Didn't expect that reaction, but that's OK."

For a president who proclaims his own success in making America "respected again" and whose turn to politics followed a grueling round of mockery at the hands of his predecessor, it's a bitter pill to swallow. And with an election year coming up, it's hard to spin world leaders laughing at you as a success - Trump himself tweeted in 2014: "We need a President who isn't a laughingstock to the entire World."

The Washington Post