For Mrs. May, who visited the White House on Friday, the goal was to clarify Mr. Trump’s position on NATO. During the presidential campaign, he derided the alliance as obsolete and questioned whether the United States would automatically come to the defense of its members. More recently, he has said the United States will support it — a pledge the prime minister claimed to have extracted from him in their closed-door meeting.

“Mr. President, I think you said, you confirmed that you’re 100 percent behind NATO,” Mrs. May said at a joint news conference, looking at Mr. Trump, who did not mention the alliance in his remarks.

Diplomats draw a distinction between Mr. Trump’s plainly false assertions about crowd sizes or that millions of people voted illegally in the election and his hints about lifting sanctions on Russia or promises to rip up the Iran nuclear deal. The first two fall into a more standard category of positions taken by a political candidate that may later change.

Mr. Westmacott said those were not what he would call “barefaced lies.”

“They are early statements of policy,” he said, “perhaps not well thought through, which European capitals — and others which might draw the wrong conclusions — need to see clarified.” Mrs. May’s meeting with Mr. Trump, he said, was a useful first step in that process.

The trouble is, Mr. Trump’s willingness to bend the truth could make it difficult to judge when the United States has settled on firm policies on these issues.

Much of that may hinge on the influence wielded by a cadre of senior cabinet officials, including Defense Secretary Jim Mattis and Secretary of State-designate Rex W. Tillerson, who hold views that differ from their boss’s and are regarded as steadier hands than him.