As President Trump departs for France to represent the U.S. at the G-7 summit of world leaders, the memories of the insults he has leveled against key U.S. allies will still be fresh, and he'll be interacting with some of those same leaders over the next few days.

In recent weeks, the president has lashed out at some of the United States' steadfast friends, while offering glowing words for strongmen like North Korea's Kim Jong Un and Russia's Vladimir Putin. In recent days, Mr. Trump has revived his suggestion that Russia should be reinstated into the group of seven leaders of major advanced economies, blaming former President Obama for allowing Russia to annex Crimea. Mr. Trump sometimes suggests America's friends treat her worse than her enemies, particularly on trade, a worldview and approach to international politics that critics say sends the wrong signal to a watching world.

"Our allies take advantage of us far greater than our enemies," the president told his audience at a manufacturing plant in Pennsylvania earlier this month.

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Earlier this week, the president unloaded on Denmark's prime minister for suggesting Greenland is indeed not for sale, describing her comments as "nasty" and suddenly canceling his trip to Denmark. Mr. Trump described his disgust in detail to reporters on the White House South Lawn Wednesday.

"No – Denmark, I looked forward to going, but I thought that the prime minister's statement – that it was absurd; that it was an absurd idea – it was nasty," Mr. Trump said Wednesday. "I thought it was an inappropriate statement. All she had to do is say, 'No, we wouldn't be interested.' But we can't treat the United States of America the way they treated us under President Obama. I thought it was a very not nice way of saying something."

He's also laid into Britain, France and Germany for not taking back ISIS fighters captured in Syria, threatening to release the fighters back into those countries.

"And, by the way, we're holding thousands of ISIS fighters right now, and Europe has to take them," Mr. Trump also told reporters on the White House South Lawn Wednesday. "And if Europe doesn't take them, I'll have no choice but to release them into the countries from which they came, which is Germany, and France, and other places."

The president has also voiced intense frustration with G-7 member France and French President Emmanuel Macron over trade-related issues, even threatening to place tariffs on French wines.

"France just put a digital tax on our great American technology companies," the president tweeted last month. "If anybody taxes them, it should be their home Country, the USA. We will announce a substantial reciprocal action on Macron's foolishness shortly."

Mr. Trump, who does not drink alcohol, added, "I've always said American wine is better than French wine!"

But it remains to be seen how Mr. Trump will handle himself in France.

At last year's summit in Canada, the president shocked attendees when he arrived late, for no given reason, to a gender equality breakfast. But he appeared to get along well with all the leaders publicly during his visit – before unleashing angry tweets after Air Force One had landed. He expressed particular ire at Canadian Prime Minister Justin Trudeau.

"Justin Trudeau of Canada acted so meek and mild during our @G7 meetings only to give a news conference after I left saying that, 'US Tariffs were kind of insulting' and he 'will not be pushed around.' Very dishonest & weak. Our Tariffs are in response to his of 270% on dairy!" Mr. Trump tweeted after the conclusion of last year's summit.