Carla Sands, the United States ambassador to Denmark, did not want Mr. Sloan to participate, and the Danish Atlantic Council “had no other option” than to revoke his invitation to speak, Lars Bangert Struwe, the secretary general of the council, said in a statement.
Mr. Sloan said the decision had left him “stunned and concerned about our country.”
On Sunday morning, Mr. Struwe canceled the NATO conference.
“After serious consideration, we have decided not to proceed with the conference,” he said on Twitter. “The progress of the process has become too problematic; and therefore, we cannot participate in the conference, let alone ask our speakers to participate.”
From a Danish point of view, the decision to bar Mr. Sloan would turn the conference’s focus to internal American politics and away from the future of NATO, Mr. Struwe said in an interview on Sunday. There were 12 people scheduled to speak, and about 100 attendees were expected, he said.
“We have all the time known that Mr. Sloan has a critical approach towards President Donald Trump,” Mr. Struwe said in the statement. “That is no secret, especially when following his Twitter and Facebook profile. We have, however, never doubted that Mr. Sloan at our conference would deliver an unpolitical and objective lecture.”