With the partisan divide in America constantly in the headlines, there’s been a lot of talk this year about the virtue of compromise. But political compromise has its limits when essential principles are involved, as Andrew Delbanco reminds readers in a new book about the tensions that led to the Civil War. He uses a case from Louisiana to evoke those troubled times, which make our present political fights seem tame by comparison.

Delbanco’s book, “The War Before the War,” revisits the decades before the Civil War, when the South embraced slavery and the North did not. That vivid difference obligated the two regions to agree to disagree, but the limits of that policy became more evident as the regions grew entwined. What law applied, for example, when a slave owner visited the North and took slaves along on the trip? That dilemma loomed large in 1836, when Mary Slater of New Orleans decided to go see her father, Thomas Aves, in Boston.

“She brought with her,” Delbanco writes, “a slave named Med, a girl of about six years old who belonged, according to the laws of Louisiana, to her husband. When Mrs. Slater became ill, she left the child with her father under his protection — or, from the point of view of antislavery Bostonians — in detention. When news reached members of the Boston Female Anti-Slavery Society that a slave girl was being held in the heart of their city, they sought a writ of habeus corpus challenging Aves’s right to keep her.”

Aves’ argued that the child legally belonged to his son-in-law, and that he was merely keeping her on his behalf. A judge eventually ruled that the child couldn’t be treated as a slave in Massachusetts, and she was given over to the care of the activists who had sought her freedom.

“The Aves case set an important precedent,” Delbanco notes. “Over the next few years, many Northern states followed it by granting freedom to slaves belonging to masters who voluntarily brought them into the state.”

Southerners were scandalized, and tensions over the issue eventually led to the clumsy Compromise of 1850, which required that fugitive slaves be returned to their masters.

The legislation was meant to preserve the Union, but it set the nation on the path to the Civil War.

Compromise on the fundamental question of whether one human could own another wasn’t possible. A painful legal dispute connected to Louisiana, Delbanco suggests, made that reality vividly clear.