• On the fourth leg of a grueling overseas trip, President Trump lectured NATO allies in Brussels on not spending enough for collective defense, and declined to plainly endorse Article 5 of the alliance’s founding treaty, which states that an attack on any member is an attack on all.

• For some of the European leaders, testing Mr. Trump seemed to be as important as finding common ground with him, amid anxiety about their relationship with a leader who had dismissed the alliance as “obsolete” and called the Belgian capital a “hellhole” after a terrorist attack.

• Earlier, Mr. Trump met with Donald Tusk, the president of the European Council, and Jean-Claude Juncker, the president of the European Commission. Mr. Tusk said afterward that there were differences of opinion over Russia, but that when it came to Ukraine, “it seems that we were on the same line.”

• Analysts said expectations were low that Mr. Trump and European leaders would agree on issues like climate change, trade and terrorism. (He also shared an eyebrow-raising handshake with President Emmanuel Macron of France.)