Secretary of State Marco Rubio traveled to Brussels on Thursday for a gathering of NATO foreign ministers amid high anxiety over the Trump administration’s approach to Europe, including the war in Ukraine, relations with Russia and President Trump’s growing trade war with the continent.

Mr. Rubio’s visit to the alliance’s headquarters, the first by a senior Trump administration official this year, comes as relations between the United States and Europe have abruptly shifted from the close cooperation of the Biden era to mistrust and acrimony under Mr. Trump.

At the same time, NATO officials may welcome a chance to confer with Mr. Rubio, whom many consider the most pro-alliance member of Mr. Trump’s national security team.

As a senator in 2023, representing Florida, Mr. Rubio cosponsored legislation requiring any president to seek the Senate’s advice and consent before withdrawing from the organization. Former aides say Mr. Trump has privately mused about taking that step, which would shatter the 32-nation military alliance formed to counter Russia.

Foreign officials who have dealt with Mr. Rubio since he became Mr. Trump’s top diplomat have described him as downplaying some of Mr. Trump’s wilder ideas and translating them into more realistic policy approaches, although they also question whether he truly speaks for a president with whom he does not have a close personal relationship.

And there is only so much Mr. Rubio can do to sugarcoat Mr. Trump’s agenda, which is driven by a view that Europe economically exploits the United States, is culturally out of sync with the values of Mr. Trump’s political movement and must do business with Russia’s president, Vladimir V. Putin.