Of course Justin Trudeau is two-faced in dealing with Donald Trump. Canadians know that. Don’t Americans know it? How can the U.S. President not know it, too?

Canadians have seen Mr. Trudeau smile and nod while sitting beside Mr. Trump as he threatened to damage the Canadian economy. Even Canadians with no love for Mr. Trudeau expect him to do his best to lather up the U.S. President with flattery when he is surprised or even appalled by what he says and does. When dealing with Mr. Trump, being two-faced is professionalism.

All foreign leaders are two-faced with Mr. Trump. He practically demands it. He threatens and bullies, but transparently craves praise no matter how insincere. The obvious tactic for any leader is to pump his ego face to face and roll your eyes when he leaves the room.

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And of course other foreign leaders gossip about Mr. Trump behind his back, in a can-you-believe-what-he’s-done-now way. They’ve never seen anything like him. He showed up at this NATO summit to berate allies for not spending enough on defence, turning photo ops into long, contentious news conferences. It’s not how politicians do things.

Prime Minister Justin Trudeau was caught on camera at a Buckingham Palace reception apparently criticizing President Donald Trump. Trump hit back on Wednesday morning calling Trudeau 'two-faced' and reiterating comments about Canada's military spending. The Globe and Mail

Mr. Trudeau was caught on camera expressing amused disbelief that Mr. Trump had turned what was supposed to be a brief handshake before their meeting into a 40-minute conference. At a Buckingham Palace cocktail party, he was wide-eyed as he told the tale: Mr. Trump, who had once planned to hold the next G7 summit at one of his own golf courses, suddenly announced it would be at Camp David, and the jaws of the President’s staff dropped to the floor.

But the other leaders who were laughing along with Mr. Trudeau – Mr. Macron, British Prime Minister Boris Johnson, Princess Anne and Dutch Prime Minister Mark Rutte – didn’t seem to be disagreeing. They talk about Mr. Trump this way.

The problem is that Mr. Trudeau was caught on camera. He went viral for leading a group of world leaders in bemused shop talk about the wacky President. That could be a big problem.

Mr. Trump doesn’t like to be challenged. At a meeting with German Chancellor Angela Merkel, the U.S. President called Mr. Trudeau “two-faced.” He then left the meeting before the final press conference, but not before he himself was caught by a live microphone saying that the “two-faced” remark about Mr. Trudeau was pretty funny.

Will there be a price to pay? Will Mr. Trump try to exact revenge, or make Canada suffer?

Maybe. Maybe not. He is the essence of unpredictability. Luckily, it is Mr. Trump, not Mr. Trudeau, who is itching to quickly ratify the new trade deal. Maybe the President will feel satisfied that he got his own shot in.

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There’s only one thing that Mr. Trudeau can do to make it better: Be as two-faced as possible.

He made a good start of it. He said he was just talking about the impromptu press conference, and quipped that every leader makes jaws drop among their staff once in a while. He emphasized that he had a “great meeting” with the President. He said he was “glad” to have a conversation about the President’s concerns.

The other NATO leaders did the two-faced thing, too. Ms. Merkel sat poker-faced while the U.S. President criticized Mr. Trudeau. The British Prime Minister, Mr. Johnson, pretended he had no idea what reporters were talking about when they asked about the overheard cocktail-party chatter.

Other leaders, such as Mr. Macron, also saw Mr. Trump commandeer photo ops into long press conferences criticizing them for spending less on defence than the NATO guideline of 2 per cent of GDP. He’s not entirely wrong. European allies aren’t entirely wrong to worry that Mr. Trump doesn’t seem to want the alliance to focus as much on Russia. Mr. Macron was right to argue when the President defended Turkey for buying Russian weapons.

For Mr. Trudeau, though, getting caught by a hot mic at Buckingham Palace isn’t likely to be a political liability here at home. Canadians spent a lot of time in 2019 debating whether this Prime Minister is, in fact, two-faced: whether he is the idealistic new-politics leader he promised to be, or a politician putting on an act.

But when it comes to dealing with Donald Trump, being two-faced is part of the job.