“Many great American soldiers, both of the North and South, died at this spot,” the inscription reads. “The casualties were so great that the water would turn red and thus became known as ‘The River of Blood.’ ”

The inscription, beneath his family crest and above Mr. Trump’s full name, concludes: “It is my great honor to have preserved this important section of the Potomac River!”…

“No. Uh-uh. No way. Nothing like that ever happened there,” said Richard Gillespie, executive director of the Mosby Heritage Area Association, a historical preservation and education group devoted to an 1,800-square-mile section of the Northern Virginia Piedmont, including the Lowes Island site…

“How would they know that?” Mr. Trump asked, when told that local historians had called his plaque a fiction. “Were they there?”