When the Texas State Board of Education met in 2010, 145 years after the Civil War ended, it decided to set new social studies standards for teaching about America’s deadliest conflict. One contentious issue before them: How central was slavery to causing the war? As the board saw it, not very.

Slavery, one board member said at the time, was “a side issue to the Civil War.” And so the board decided on new standards. The state’s roughly 5 million students would be taught that the cause of the war was “sectionalism, states’ rights and slavery.”

The decision to marginalize the role of slavery by listing it third was attacked by many, including historians and social studies teachers. They pointed to overwhelming evidence that the South seceded precisely because it worried that Abraham Lincoln’s election in 1860 would mean a push to end slavery. Last year, after much wrangling, the board amended its standard to put slavery at the forefront.

But the fight over the language in education standards points to a bigger issue regarding the teaching of slavery in U.S. history. Every state — and almost every school district — does it differently. Unlike with math and science, there is no nationally agreed upon set of standards for teaching social studies. What public school children in the United States learn about slavery has almost everything to do with where they grow up.

In their official standards for teaching social studies and history, some states explicitly call for teaching about aspects of slavery throughout a student’s K-12 education, while others refer to it in passing or not at all.

Massachusetts mentions slavery 104 times in its history and social studies framework. Louisiana’s standards for K-12 social studies refer to slavery four times. Idaho’s guidelines mention slavery twice. Few states mention the enslavement of Native Americans in their standards despite growing scholarship that points to it being widespread in early colonial America and continuing throughout much of the 19th century, particularly in Western states and territories.

In practice, individual school districts take guidance from the states and then decide on the course work for their students. In states that demand extensive teaching about slavery, such as Massachusetts, school districts must meet those requirements. But even in states where guidelines don’t require a fuller knowledge of slavery, districts can choose to teach about it in-depth.

For Tiffany Classie Williams, a history teacher in Birmingham, Ala., who is African American, educating students about slavery goes far beyond the 15 mentions of slavery in the Alabama guidelines. For Williams, it’s personal. And it’s an opportunity to instill in her students, almost all of whom are black, a sense of resolve and pride.

“I’m a descendant of slaves, obviously, and so I want to teach it and teach it well,” said Williams, who is black and has been teaching at Huffman High for 12 years. “I want my students to be resilient. I’ll say, ‘Don’t forget, your ancestors survived the Middle Passage.’ And if they survived the Middle Passage, then, baby, you can do anything.”

In Philadelphia’s public school district, students must take a year of African American history in high school, a requirement that makes the district a rarity in the United States. The class follows the experience of African Americans beginning with a section on life in Africa before the transatlantic slave trade took about 12 million people to the Americas. The course also covers the experience of slavery in America beginning in 1619 through to the Civil War, Reconstruction, Jim Crow and the civil rights era.

Mark Hoey teaches Advanced Placement American history to juniors at the Philadelphia High School for Girls. Students in his class have taken the required African American history course the year before and are well-versed in its lessons.

“My kids are really strong in that,” Hoey said. “It’s one area where I know they’ll have an insider’s, expert opinion.”

For some historians and educators, such as Daina Ramey Berry, a University of Texas history professor and author of “The Price for Their Pound of Flesh: The Value of the Enslaved, From Womb to Grave, in the Building of a Nation,” one option for redressing the lack of knowledge about slavery would be to focus an entire course on the subject in middle school or high school.

“It is such a major part of American history, but it gets glossed over. I mean, the foundation of our early economy was based on slavery, and that’s missing,” Berry said. “And also, it’s the foundation of some of the challenges we have with current race relations. We never healed from this. It’s like that elephant in the room of our history that people don’t want to talk about because it’s uncomfortable.”

But even teachers who want to focus more on slavery find they’re often pressed for time.

“History, especially in younger grades, gets kind of pushed aside for testing,” said Jordan Dunkerson-Hurst, who teaches American history at Ernest Childers Middle School in Broken Arrow, Okla., a suburb of Tulsa. “And so even if you are a person who wants to go deep into the real stuff, sometimes you’re forced to just skim over to get what you need to get done in the time you’re allotted.”

But Dunkerson-Hurst, a member of the Choctaw tribe, doesn’t skim when it comes to teaching her students about the role slavery has played in America. And she notes that her tribe, and others in Oklahoma that had been forced west on the Trail of Tears by President Andrew Jackson, also kept African Americans as slaves.

“I always tell them … when we learn about history, we stop repeating our mistakes,” she said. “And so my goal isn’t really just to teach history. It’s to create better people to some degree.”

While some states and school districts have made strides in addressing the dearth of knowledge about slavery and placing a greater emphasis on studying it, the approach has been inconsistent and haphazard. There are few textbooks or lesson plans that adequately address its scope and impact. And, for now at least, there remains a societal lack of understanding about how dramatically and unrelentingly slavery has shaped almost every aspect of America’s history — and its present.

That’s a problem, say educators and historians. They believe the history of slavery most U.S. students are learning doesn’t begin to capture the enormity of the practice or how it continues to affect the country today.

Reporter Nicole Ellis visits Africatown, Ala., for the commemoration of the Clotilda, the last known slave ship to arrive on U.S. shores. (The Washington Post)

The failure to educate Americans about slavery in a deep and unflinching way reinforces divisions, said Bethany Jay, an associate professor at Salem State University and co-editor of “Understanding and Teaching American Slavery.”

The nation finds itself riven by divisive issues such as Confederate statues, kneeling athletes, prison reform and birthright citizenship because, Jay said, there is a “lack of context for how all those issues tie into slavery itself or its immediate aftermath. And that’s the fault of how we’ve been talking about this issue in our schools and our museums and our public life and culture.”

Historians and teachers call for a national embrace of standards that fully address how slavery shaped America’s economy, was encoded in its laws, protected by courts and expanded by national policy even as other countries were banning the practice and abolitionists were calling it immoral. They want a more explicit teaching of how American slavery was justified by racist beliefs.

In a report last year titled “Teaching Hard History,” the Teaching Tolerance project of the Southern Poverty Law Center examined 15 sets of state standards to see how well they addressed 10 key concepts related to slavery. Those concepts included teaching that slavery existed in all 13 colonies, that protections for slavery were embedded in the founding documents and that slavery and the slave trade were central to the development and growth of the U.S. economy. According to the report, the state standards rarely addressed the key concepts in any meaningful way.

“The point is not to teach American history as a chronicle of shame and oppression. Far from it,” Yale historian David Blight, author of “Frederick Douglass: Prophet of Freedom,” wrote in the report’s introduction. “The point is to tell American history as a story of real human beings, of power, of vast economic and geographical expansion, of great achievements as well as great dispossession, of human brutality and human reform.”

For now, many teachers and historians say, America’s public schools are failing to meet that standard.

About the project: Teaching Slavery

For this project on how slavery is taught, The Washington Post interviewed more than 100 students, teachers, administrators and historians throughout the country and sat in on middle school and high school history classes in Birmingham, Ala.; Fort Dodge, Iowa; Germantown, Md.; Concord, Mass.; Broken Arrow, Okla.; and Washington, D.C.

The articles in this project examine the lessons students are learning about slavery, obstacles faced by teachers in teaching this difficult subject, the right age to introduce hard concepts about slavery to young students and how teachers connect the history of slavery to 21st-century racism and white supremacy. Our focus is on public schools because teaching choices are made by elected policymakers and school officials who determine curriculum and whose decisions are implemented by administrators and teachers whose salaries are publicly funded. Find other stories from the project here.