For more than a century, the most popular U.S. history textbooks portrayed slavery and the oppression of nonwhite people in a way that reinforced white supremacy, a new book argues.

In They Were White and They Were Good, Harvard historian James Oakes shows how textbooks, starting in the late 19th century, emphasized that white Americans were responsible for “civilizing” conquered territories and marginalized groups, from Native Americans to African Americans to Mexican Americans.

The books downplayed the role slavery and colonialism played in the nation’s history, and often omitted accounts of white violence against minority groups, according to Oakes.

Oakes writes that even relatively progressive history texts of the 20th century largely ignored such historical events as lynchings and the massacre of Native Americans.

“Textbooks were written by white people, for white people,” Oakes told The New York Times in an interview. “They celebrated the history of white people as a story of progress and liberty.”

Oakes found that the books often omitted or downplayed historical facts that didn’t fit their narrative of white supremacy. For example, he cites one 1920s textbook that mentioned the Battle of Little Bighorn, in which Sioux warriors under Chief Sitting Bull defeated U.S. soldiers led by George Custer, in less than 10 words.

As Oakes wrote, “The whitewashing went well beyond history textbooks. According to the American Historical Association, history was the most popular subject in schools at the end of the 19th century. By 1925, it had dropped to sixth place.”

Oakes says the textbooks had a profound impact on the teaching of history and the development of racial attitudes in the United States.

“They helped to shape a national identity that was based on white dominance,” he said.

Oakes also argues that the way history is taught in schools today continues to reflect this white-supremacist legacy.

“Textbooks are still often sanitized versions of history that downplay the role of racism and oppression,” he said. “We need to challenge this narrative and tell a more inclusive and accurate history of the United States.”