Secretary of State Antony J. Blinken visited NATO headquarters in Brussels on Wednesday at what he called a “critical moment” for Ukraine and the U.S.-led military alliance, as Europe braces for the anticipated upheaval of a new Trump era in Washington.

In a trip organized only after last week’s presidential election results made clear that U.S. policy will likely swing dramatically away from President Biden’s lock step support for NATO and Ukraine, Mr. Blinken met with alliance and European officials to help plan for a post-Biden future.

Donald J. Trump’s return to the White House in January has deeply shaken Europe’s mainstream political leaders, thanks to his skepticism about the value of NATO, the cost of defending Ukraine, and the wisdom of isolating Russia and its president, Vladimir V. Putin.

Once in office, Mr. Trump could move quickly to change U.S. policy on all three fronts — a shift that European leaders fear might leave their countries both less secure from Russian aggression and at an economic disadvantage.

Mr. Blinken did not explicitly mention Mr. Trump or last week’s election in his public remarks after meetings at NATO headquarters. But an American leadership change with huge global import was the obvious subtext, as Mr. Blinken stressed the intrinsic value of the alliance.

Ukraine’s foreign minister, Andrii Sybiha, addressed the elephant in the room before sitting down with Mr. Blinken at a Brussels hotel. He said that their meeting offered “an opportunity to coordinate steps” after the U.S. election, noting that Ukraine’s government was speaking “both with the president-elect and his team and also with the outgoing administration.”