The long-awaited, historic meeting between President of the United States Donald Trump and Russian leader Vladimir Putin will take place in Helsinki, Finland, on June 16, 2018. As CNN reported, citing sources familiar with the matter, the POTUS is planning a one-on-one meeting with the Russian leader.

The Helsinki meeting will mark the first formal summit between the two leaders. The effects of the impending summit can already be felt across the pond. According to Reuters news agency, German lawmakers and security experts have expressed concern, unanimously warning of the repercussions Donald Trump’s potential circumventing of European allies and NATO members could have.

“There are great concerns in the alliance about what agreements Trump and Putin could reach,” transatlantic coordinator for Chancellor Angela Merkel’s ruling coalition Peter Bayer told state media, adding that NATO members have not been included in the planning of the Helsinki summit.

Comparing Trump’s meeting with Putin to the president’s recent meeting with North Korea’s Kim Jong-un, Bayer stated that the meeting was not really a tremendous success as the POTUS is suggesting, adding that Kim has “only made promises thus far.” Trump could let Putin “put one over on him,” in Helsinki, Bayer suggested.

As Reuters notes, Donald Trump is set to leave for Europe next week, where he will hold meetings with NATO allies, and visit Britain. Some German lawmakers think Trump could mirror what he did at the G7 summit, and refuse to sign a communique.

This option, according to Wolfgang Ischinger, head of the Munich Security Conference and a former German envoy to Washington, “cannot be ruled out.”

Contradicting Trump’s criticism, Ischinger added that NATO is in its best shape in years, given that a substantial increase in military spending on countering Russia via Poland and the Baltic states had required significant effort, financial and otherwise, on NATO’s part.

Christian Lindner, head of the pro-business Free Democrats, called Donald Trump “volatile,” stating that the president is capable of changing his position completely within 24 hours. Lindner accused Trump of jeopardizing the relationship between Europe and the U.S. through his actions on trade and security, and added that the president is responsible for the growing anti-American sentiment in Germany.

Still, Lindner said, the two countries will remain close allies. However, a united European stance against Putin is, head of the Germany’s Free Democrats suggested, a must.

At this year’s G7 summit, Donald Trump floated the idea of re-inviting Russia to rejoin the organization, prompting more speculation about what looks like a tendency of alienating allies — through trade tariffs, political maneuvers, and rhetoric — and catering to the interests of adversaries, one of which is Russia. As Dallas News‘ Carl P. Leubsdorf recently noted, Trump’s pattern of cozying up to Russia might jeopardize NATO interests, and benefit Putin.