The combination of a high-stakes NATO summit and a one-on-one meeting between President Donald Trump and his Russian counterpart Vladimir Putin could bring about the most dramatic geopolitical sea change since the end of the Cold War, analysts have told CNBC.

The U.S. president’s working trip to Europe started with a two-day international gathering of NATO members in Brussels on Wednesday, with leaders hopeful of projecting solidarity in the face of Russian threats to divide the group.

Yet, shortly after sitting down with NATO chief Jens Stoltenberg on Wednesday morning, Trump began ruffling transatlantic feathers — again.

“Germany is totally controlled by Russia… They will be getting between 60 and 70 percent of their energy from Russia and a new pipeline, and you tell me if that is appropriate because I think it’s not,” Trump said, before criticizing Berlin’s failure to significantly increase defense spending.

In response, Germany’s Defense Minister Ursula von der Leyen told CNBC she believed Berlin could “cope” with Trump’s repeated criticisms.

The mood music of this year’s NATO summit is contrasting sharply to previous international gatherings. They were once seen as relatively routine affairs, with member nations uniting to say the alliance had never been stronger and all pledging to continue to work together.

Nonetheless, while envoys have sought to carefully negotiate critical components of the Brussels communique ahead of time, Reuters reported that one NATO member said the hard work could prove pointless if the U.S. president then nullifies everything with a tweet.

“Donald Trump heads into the high-stakes NATO summit sounding an awful lot like Vladimir Putin,” Brian Klaas, a fellow in global politics at the London School of Economics, told CNBC via email.

He has long complained that NATO members are not meeting the alliance’s defense spending goal, although expectations of significant progress over this issue are not particularly high. That’s because the broader U.S.-European relationship has already been soured by Trump’s trade tariffs, his withdrawal from the Iran nuclear deal and a rocky first meeting with NATO leaders last year.

“The global order is underpinned by alliances and institutions that Trump seems determined to destroy,” Klaas said, before adding: “If he doesn’t stop praising adversarial dictatorships who seek to harm the United States while savaging America’s historic democratic allies, then we may be in the midst of a global shift in geopolitics that has the potential to be more consequential than any foreign policy shift since the end of the Cold War.”

This year’s NATO meeting comes ahead of Trump’s direct meeting with Putin in Helsinki, Finland, on Monday.

Speaking to supporters ahead of his European trip at a rally in Montana last Thursday, Trump said: “Trust me, we’ll be fine… Will I be prepared? Totally prepared. I’ve been preparing for this stuff my whole life.”

The Helsinki summit comes despite Europe’s policy of trying to isolate the Putin from the global stage, while Britain — Washington’s key ally — has also expressed concern over what they see as Trump’s overly friendly attitude toward the Russian leader.

Since becoming president in January last year, Trump has sought to improve ties with Moscow — even as Washington tightens economic sanctions.

“A repeat of the June G-7 summit fiasco would weaken Trump’s position and tempt Putin to strive to widen the emerging differences between the United States and its European allies,” Steven Pifer, a former U.S. ambassador to Ukraine under President Bill Clinton, said in a blog post published last week.

“In contrast to his tweets, Trump shies away from confrontation in face-to-face meetings. If he ducks or soft-pedals difficult issues such as Ukraine or election interference, Putin will take Trump for a sucker and play him accordingly.”