BRUSSELS — Surrounded by stone-faced allies, President Donald Trump rebuked fellow NATO members Thursday for failing to meet the military alliance’s financial benchmarks, asserting that leaves it weaker and shortchanges “the people and taxpayers of the United States.”

Trump, who has often complained back home about other nations’ NATO support, lectured the other leaders in person this time, declaring, “Many of these nations owe massive amounts of money from past years.”

The president’s assertion immediately put NATO under new strain and did nothing to quiet questions about his complicated relationship with an alliance he has previously panned as “obsolete.” Notably, he also did not offer an explicit public endorsement of NATO’s “all for one, one for all” collective defense principle, though White House officials said his mere presence at the meeting signaled his commitment.

Fellow NATO leaders occasionally exchanged awkward looks with each other during the president’s lecture, which occurred at an event commemorating the fall of the Berlin Wall and the Sept. 11, 2001, terrorist attacks. When Trump tried to lighten the mood with a joke about NATO’s gleaming new home base — “I never asked once what the new NATO Headquarters cost” — there was no laughter from his counterparts.

NATO officials had expected Trump to raise the payments issue during Thursday’s meeting, even preparing Secretary-General Jens Stoltenberg for the prospect that the president could try to pull off a stunt like handing out invoices. But one European official said NATO members were still taken aback by the aggressive tone of his speech.