In an interview with The New York Times just before officially claiming the Republican nomination last July, Mr. Trump said that if he were elected, the United States would come to the defense of the Baltic States against a Russian invasion only if those small countries spent more on their military and contributed more to the alliance.
“If they fulfill their obligations to us,” Mr. Trump said in the interview, “the answer is yes.”
Mr. Trump’s speeches often remain in flux until the last minute, and he is well known for deviating from his prepared text by adding, removing or changing passages even as he reads them on the prompter. So he may yet change his mind and omit an explicit statement of support for Article 5.
But the administration official said that Mr. Trump now appears ready to reassure NATO allies that the United States will not place conditions on its adherence to Article 5, which states the principle that an attack on any one member is an attack on all.
European leaders have feared a historic American retreat from the collective defense pact that created the NATO alliance, signed by President Harry Truman 68 years ago in the wake of World War II. They worried, in particular, that Mr. Trump’s silence on Article 5 was inviting further aggression from President Vladimir V. Putin of Russia, whose troops seized Crimea from Ukraine in 2014 and have helped destabilize eastern Ukraine since then.
“It could raise grave doubts about the credibility of the American security guarantee and provide Russia with an incentive to probe vulnerable Baltic States,” Thomas Wright, a Brookings Institution scholar, wrote this week, before Mr. Trump began his first foreign trip as president.
The NATO leaders who will meet on Thursday face other difficult questions as well, including how many troops the…