As President Donald Trump prepares to head to Europe, he is snubbing close allies while reaching out to rivals.

The president abruptly cancelled a planned trip to Denmark Tuesday after his proposal to purchase Greenland was mocked as "absurd."

That same day, Trump again suggested he would like to see Russia rejoin the G7, a group of the world's leading economies that the country was expelled from for annexing Crimea from Ukraine.

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President Donald Trump is snubbing a close US ally while reaching out to a top US rival as he prepares to head to Europe.

On Tuesday, Trump abruptly cancelled a planned trip to Denmark after his proposal to purchase Greenland was dismissed as "absurd."

That same day, he suggested that Russia be re-instated as a member of the Group of Seven (G7), a collection of industrialized nations formerly known as the G8 until Russia was expelled in 2014 in retaliation for to its annexation of Crimea from Ukraine. In a break with some US allies, Trump supports Russia's re-instatement without any conditions.

These developments come as Trump prepares to travel to Europe this week for the G7 summit in Biarritz, France.

The Danish prime minister said Trump's apparent interest in purchasing the autonomous territory was "absurd," explaining that "Greenland is not for sale."

In response to the prime minister's remarks, Trump tweeted, "Denmark is a very special country with incredible people, but based on Prime Minister Mette Frederiksen's comments, that she would have no interest in discussing the purchase of Greenland, I will be postponing our meeting."

The prime minister expressed "regret and surprise" at Trump's decision.

"I think it's sad, honestly, because this is just not the way you treat an ally," former US ambassador to Denmark during the Obama administration Rufus Gifford told CNN.

The former ambassador added that he "had the great responsibility of going to the Danish government and requesting troops to go to Iraq, to Syria. And they went and they fought alongside our troops and they died alongside our troops."

Denmark has been a steadfast US ally, one that has played a role in impeding progress on the Nord Stream 2 natural gas pipeline between Russia and Germany, a project to which the Trump administration is firmly opposed. Denmark has repeatedly caused headaches for Russia in this area.

As Trump insulted Denmark, he made the comment that it would be "much more appropriate" to have Russia back in the G7. "I could certainly see it being the G8 again," he told reporters in the Oval Office Tuesday.

The president suggested that the reason Russia was expelled was because Russian President Vladimir Putin "outsmarted" former President Barack Obama, who he said apparently "thought it wasn't a good thing to have Russia in" and "wanted Russia out."

Russia was added to the G7, which then became the G8, when the US, the UK, Canada, France, Germany, and Japan made the decision to include it in 1998. It was kicked out 16 years later in order to impose a cost on Russia for its actions in Ukraine.

Trump pushed for Russia to be let back into the group last year. "Russia should be in this meeting," he said. "They should let Russia come back in because we should have Russia at the negotiating table."

This is a pattern of behavior by Trump that has been seen over and over again. Earlier this month, for instance, the president made the decision to side with North Korean leader Kim Jong Un over long-time US ally South Korea. As the US and South Korea conducted joint drills, Trump said he didn't care for the joint military exercises designed to strengthen the US-South Korean alliance — drills North Korea sharply criticizes.

Read more: Trump again sides with Kim Jong Un over long-time ally as North Korea fires off more missiles

"I never liked it. I don't like paying for it," Trump explained, adding, "We should be reimbursed for it."

The president regularly criticizes allies and partners for not paying what he considers to be their fair share for US defense commitments, which Trump characterizes as one-sided deals. Throughout his presidency, long-standing alliances have been treated less as valuable partnerships and more as business transactions.

Tweeting Wednesday, Trump again slammed NATO allies for not paying enough for collective defense, calling the situation "very unfair to the United States."

In separate tweets, he criticized Demark, a "wealthy country' for only spending 1.35 percent of its GDP on the NATO alliance.

"We protect Europe and yet, only 8 of the 28 NATO countries are at the 2% mark," he said, stressing that countries like Denmark are "still way short of what they should pay for the incredible military protection provided."