BRUSSELS — For an anniversary bash, the guests are unusually anxious.

The leaders of NATO are traveling to London this week to commemorate the alliance’s 70th birthday — but they will do so at a carefully crafted, foreshortened gathering, not at a full-blown summit meeting.

The low-key celebration seems intended to avoid more awkward comments from President Trump, who nearly blew apart the last summit meeting in Brussels in July 2018, musing about quitting the alliance and walking out in the middle of a statement by Chancellor Angela Merkel of Germany.

Normally, the 70th anniversary, like the 50th, would have been held in Washington, where the alliance’s founding treaty was signed, with three days of pomp, substance and a White House dinner. But given Mr. Trump’s unpredictability and his doubts about the alliance, NATO countries decided to have only a foreign ministers’ meeting in Washington on the actual anniversary, in April.

Despite some misgivings, including from Berlin, this session in London was added because Britain wanted to prove that it still mattered in trans-Atlantic security, particularly with its withdrawal from the European Union looming, said Jonathan Eyal , assistant director of the Royal United Services Institute, a defense research institution based in London.