James W. Loewen, a sociologist and civil rights champion who took high school teachers and textbook publishers to task for distorting American history, particularly the struggle of Black people in the South, by oversimplifying their experience and omitting the ugly parts, died on Thursday in Bethesda, Md. He was 79.

His death, in a hospital, was confirmed by Ellen Adler, his publisher at the New Press, who said he died after an unspecified “long illness.”

“Those who don’t remember the past are condemned to repeat the 11th grade,” Dr. Loewen wrote in “Lies My Teacher Told Me: Everything Your American History Textbook Got Wrong” (1995), the best known of his dozen books attacking historical misconceptions.

Dr. Loewen was a relentless contrarian who challenged anyone who imagined academic life as a passage through genteel lectures on settled matters for drowsy students on leafy campuses. He charged through history like a warrior, dismantling fictions and exposing towns for excluding minorities; teachers and historians for dumbing lessons down; and defendants in 50 class-action lawsuits who, according to his expert testimony, victimized people in civil rights, voting rights and job discrimination cases.

A Northerner fascinated with Mississippi, he wrote his first book about the Chinese population there. He wrote another about how America’s historic sites distort our knowledge of the past. And it was a mistake to get him started on the origin of Thanksgiving: Plymouth was already a village with cleared fields when the Pilgrims found it deserted by plague victims. No turkey was served in 1621 — perhaps it was duck. And there was no pie. The settlers had no wheat flour for crust and no oven for baking. The holiday Americans celebrate has nothing to do with the Pilgrims. It was invented 242 years later by Abraham Lincoln to celebrate the North’s victory at Gettysburg.

“History is by far our worst-taught subject in high school,” Dr. Loewen told The Atlantic in 2018. “I think we’re stupider in thinking about the past than we are, say, in thinking about Shakespeare, or algebra, or other subjects. Historians tend to make everything so nuanced that the idea of truth almost disappears.”