“The whole notion of honoring the Confederacy and the sacrifice that your family made became part of what we taught in the schools,” said Charles Dew, a Williams College historian whose book “Apostles of Disunion” describes the white supremacist arguments that underpinned the South’s case for leaving the Union.

To correct the record, Mr. Dew gave talks about his book in the early 2000s, as part of the National Park Service’s efforts to clarify the causes of the war. Some audiences pushed back, saying, “My family did not own slaves, so how could they have been fighting for slavery?”

“I’m not trying to denigrate your ancestors,” Mr. Dew said he told those people. “I’m trying to explain why the war came and ask everyone to consider the issue with an open mind.”

In Russia, people choked on memory. As the Soviet Union was falling, the sins of the past flooded the present. Newspapers wrote about Soviet repressions. Researchers began documenting political killings. All this, as Russians were losing their jobs, their savings, their respect in the world and their dignity. They could not afford to lose their past. So Stalin became the man who led the Soviet Union to victory in World War II and industrialized a peasant nation.

Germany is the exception. It took a generation, but German society faced its Nazi past and has emerged as an exemplary democratic culture. That is partly because of the extreme nature of the Nationalist Socialist regime and the devastation of the war it brought. Germans had to come to terms with the Holocaust. The Nazi regime had perhaps overcome the depression, but its increasing brutality left no redeeming qualities.

“In the United States, slavery was embedded in a constitutional regime that at least verbally offered universalizing rights,” said Charles Maier, a professor of 20th-century history at Harvard. “The German cause had no equivalent.”

But while top Nazis were put on trial right after the war, with the world watching, mainstream German society did not fully grapple with the crimes until the 1960s. There was a political shift to the left that encouraged young Germans who posed hard questions about their parents’ past. Even today, there are no major memorials to the perhaps half a million Germans who died in Allied bombing campaigns in Hamburg, Dresden and other cities, as that would be seen an assertion of equivalence.