Mr. Trump appears to have a special animus toward Germany, believing that Berlin has developed a vibrant social system and thriving export-driven economy unfairly, and on the back of the United States, by not spending enough on defense.

At a campaign rally last week in Great Falls, Mont., Mr. Trump previewed his trip to Brussels to thousands of supporters. “I’ll see NATO and I’ll tell NATO, ‘You’ve got to start paying your bills,’” he said. “The United States is not going to take care of everything.”

He also said he had suggested to Chancellor Angela Merkel of Germany that the tens of thousands of United States troops who are stationed in her country might not be worth the expenditure — an opinion he has shared privately with advisers at the White House, according to one person familiar with the discussions.

Last month, Mr. Trump wrote personal letters to the leaders of several NATO allies, taking them to task for failing to live up to a goal set in 2014 that every member work toward spending 2 percent of its gross domestic product on defense. His note to Ms. Merkel was particularly pointed, holding Germany responsible for other allies’ shortfalls as well as its own.

Germany has promised to increase military spending to 1.5 percent of its economy by 2024. While not the 2 percent level, Berlin will argue that will still be more than any other NATO country other than the United States.

Norbert Röttgen, chairman of the German Parliament’s Foreign Relations Committee, said the answer to Mr. Trump is “to accept he has a point, and respond by displaying more European strength and enhancing European defense in cooperation with NATO.” As a whole, the alliance’s European members spend about $200 billion a year.

“That’s a lot, but it’s cost inefficient, militarily ineffective and lacks political weight and impact,” he said. “We need to strengthen the European pillar of NATO.”