On 14 September 1874, less than a decade after the end of the US civil war, the former Confederate general James Longstreet was back in arms. This time, he was seeking to prevent an insurrection: a white supremacist bid to take over New Orleans.

Once seen by northerners as among the three most notorious Confederates – with his commander, Robert E Lee, and president, Jefferson Davis – Longstreet now led state militia and city police. His troops were Black and white, reflecting an unlikely commitment to post-war civil rights that would waver in later years. His complex life is the subject of a new biography, Longstreet: The Confederate General Who Defied the South, by Elizabeth R Varon, a history professor at the University of Virginia, currently the visiting Harmsworth professor of history at Oxford.

“This turnabout is so fascinating,” Varon marvels. “I pitched the book as the story of the most remarkable political about-face in American history.”

An enslaver, Longstreet directed Confederate forces to capture Black people and take them south to slavery or imprisonment. He fought until the surrender at Appomattox, then allied himself with those who had brought about his defeat: Ulysses S Grant and the Republican party.

“He was not the only one,” Varon says of white southern Republicans who made such moves, “but [he was] the highest-ranking Confederate. He was a lightning rod for critics.”

Prominent figures such as Lee were honored with monuments, some of which have recently been pulled down. Longstreet never had this problem, because you’d be hard-pressed to find such tributes to him.

“It’s quite astounding,” Varon reflects. “Longstreet endorses Reconstruction at a time when the vast majority of white southern former Confederates pledged themselves to resist at all costs.”

The author is interested in such dissenters. A previous book chronicled Elizabeth Van Lew, a resident of the Confederate capital, Richmond, who spied for the Union. Varon hopes a future scholar will write about another dissenter, Longstreet’s much younger second wife, Helen Longstreet, née Dortch, who outlived her husband by 58 years. By the 20th century, she was also an outspoken voice for civil rights in the south.

Dissent characterized Longstreet’s war years as much as his later life did. The 1993 film Gettysburg dramatizes his dispute with Lee at that famous battle. Longstreet argued for a defensive approach. Lee took the offense and the result was a disaster, a turning point in the war. Transferred west, Longstreet led an assault credited for the victory at Chickamauga, then lambasted his new commander, Braxton Bragg, for his failure to capitalize. Longstreet would later suffer for daring to criticize Lee.

Although Varon addresses Longstreet’s war years, she is more interested in his postwar career, which stretched for nearly four decades and included leadership positions in Louisiana and Georgia. He even became the US minister to the Ottoman empire, where he met Sultan Abdul Hamid II and defended American missionaries.

He owed much of his success to an improbable allegiance to the abolitionist Republican party of Abraham Lincoln and a lasting friendship with Grant.

Varon details an unconventional but unsuccessful peace initiative involving the Grant and Longstreet families near the end of the civil war. (The war years had been hard for Longstreet and his first wife, Louise Longstreet. They lost three children to scarlet fever in 1862, and two years later, the general was grievously wounded by his own men.) At Appomattox, Longstreet was impressed by Grant’s lenient terms, which helped convince him it was time to change. He explained his stance in a series of 1867 letters that were poorly received by many.

As Varon explains: “Longstreet said, ‘Yes, let’s give the Republican party a chance, try to make this work, we appealed to arms and the sword to arbitrate the political conflict with the north, they won, now it … requires me to try to make the best of it.’”

She adds: “He was absolutely thrown back on his heels by the backlash by ex-Confederates. For his willingness to work with the Republicans, he was called anathema, a Judas, Lucifer, Benedict Arnold, they wished he’d died during the war.”

A new battle began, a war of words with fellow former commanders such as Jubal Early, over who was responsible for the defeat. Yet Longstreet was committed to Reconstruction and the Republicans and to his postwar home, New Orleans, a racially diverse city where he held political positions following Grant’s election as president in 1868, beginning at the customs house. Through such positions, which extended to militia and police leadership, Longstreet advocated some degree of civil rights. Allies included PBS Pinchback, who in 1872 became the first sworn-in Black governor of a US state.

In addition to Longstreet’s personal life and recognition of the flawed rebel war effort, Varon identifies “the last element” in his turnaround as “New Orleans itself – a unique political environment”. She cites the city’s Afro-Creole male leadership class, many of whom served as officers in the Union army.

“They were politically savvy, assertive men,” Varon says, “really pushing for votes and full civil, economic and social rights for Blacks in Louisiana.” Regarding Longstreet, she notes: “I don’t think it would have turned out the same if [he] was somewhere else in the postwar south. This particular setting was uniquely positioned to change his views on race.”

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By 1874, that change was profound. On George Washington’s birthday, Longstreet participated in a review of interracial troops. Racist white discontent was simmering, in part over a disputed election two years earlier: after the Republicans were declared to have won, Democrats set up a rival government, followed by a takeover attempt and a massacre of Black people at Colfax. Another slaughter of Black people followed, in Coushatta in the summer of 1874. That fall, a group called the White League led a march on New Orleans.

The insurrectionists targeted government property and overwhelmed authorities. Longstreet was wounded in the so-called Battle of Liberty Place, which ended with the rioters in control of the city. Their three-day takeover ceased with the approach of federal forces but the riot spelled doom for Reconstruction in Louisiana, presaging the demise of the policy throughout the southern states.

Longstreet’s subsequent life brought something of a retrenchment on civil rights. Relocating to Georgia, he maintained ties to the Republican party but focused on cultivating white support. He also pursued two significant projects – restoring national bonds ruptured in the civil war, and defending his Confederate career, in part through a near 700-page autobiography.

“He focuses on setting the record straight and answering charges as he gets older,” Varon says. “He claws back some of his lost popularity among white southerners. He reinvents himself as a herald of reconciliation. Both sides are going to have to make concessions.”

As a US marshal, Longstreet did prosecute white supremacists and continue to back voting rights for all eligible citizens.

“He remains kind of enigmatic,” Varon reflects. “In the last years of his life, he tries to reconcile his Confederate and Republican identities. It was not possible to ever fully do that.”

• Longstreet: The Confederate General Who Defied the South is published in the US by Simon & Schuster