The United States ambassador to Denmark barred an American NATO expert critical of President Trump from speaking at an international conference hosted by the American embassy and a Danish think tank, prompting the event’s cancellation, organizers said.

The expert, Stanley R. Sloan, was scheduled to give a keynote speech at the conference, which was celebrating the 70th anniversary of NATO, on Tuesday.

Mr. Sloan, a visiting scholar at Middlebury College in Vermont, a fellow at the Atlantic Council and a former analyst for the Central Intelligence Agency, planned to speak about the future of trans-Atlantic relations.

One day before he was set to leave for Copenhagen, Mr. Sloan was informed that the United States Embassy in Copenhagen had vetoed his participation because of his previous criticisms of President Trump, Mr. Sloan said on Facebook on Saturday.