Gregory Downs is a professor of history at the University of California, Davis, and the author of the forthcoming “The Second American Revolution: The Civil War-era Struggle over Cuba and the Rebirth of the American Republic.”

Everyone knows that Confederates fought the Civil War to preserve and extend the slave system that produced their wealth and shaped their society. But what, exactly, did white Northerners fight for? In her often-riveting “Armies of Deliverance,” Elizabeth R. Varon answers that question in a new way, with important ramifications for how we understand the nation’s most significant conflict, the meaning of anti-slavery politics and the disappointments of postwar Reconstruction.

Because Confederates launched the first assaults of the Civil War, and because Confederates so eagerly trumpeted their defenses of slavery, Northern motivations can seem irrelevant. Confederates attacked the United States, and the United States fought back. Yet historians have debated Northern motivations vigorously over the past few decades, because those motivations tell us a good deal about why the Civil War came, what kind of war it was and what its impact would be upon U.S. society. One loosely defined group of historians argues that most white Northerners aimed primarily to restore the Union: to preserve the nation and not to transform it. Other historians, meanwhile, claim that white Northerners generally sought to extend freedom by creating a new nation without slavery. The answer turns on which Northerners one examines — common soldiers, female teachers and nurses, free black activists, Ohio Valley politicians, officers in high command — and how one evaluates inherently slippery evidence about motivation.

This debate has real ramifications for how we understand the Civil War era. Did secessionists have genuine reason to fear white Northern intentions? Was the war restrained, or did it approach a total war? And did the Civil War fundamentally transform the lives of the 4 million enslaved Americans and undermine the nation’s foundations in white supremacy? Historians who emphasize the desire to restore the union generally argue that secessionists miscalculated white Northern intentions and that many white Northerners saw their job as returning, not remaking, white Southerners — even secessionists. Thus, they argue, white Northerners favored restraint during and after the Civil War to ease the reintegration of white Southerners. Those historians who emphasize the freedom story are more likely to see Southern secession as a reasoned response to transformative Northern goals, to trace increasingly bold war measures and to narrate ambitious plans for national re-creation in Reconstruction.

The argument between scholars on either side of the union and freedom debate is important but in danger of becoming repetitive. So it is a relief to watch Varon strive elegantly to escape that binary perspective and establish her own interpretive framework for white Northern motivations. Her answer is deliverance. Christians, North and South, looked to biblical stories of deliverance to explain how society could be transformed. For Confederates, deliverance was simple: They would be delivered from the tyranny of Northern political opinion. Enslaved people similarly saw deliverance in stark terms: escape from the tyranny of masters.

But how did white Northerners understand deliverance? Varon argues that many of them believed that white Southerners needed deliverance from their “scheming leaders,” the despotic planters who shut down public debate and dominated the political system. Once freed, the great mass of white Southerners would begin to think for themselves and, ineluctably, emulate the prosperous and free North. White Southerners’ political independence would then free the nation from the sway that planters exercised over politics and policy, a sway Northerners denounced as a despotic slave power. Deliverance, Varon writes, “resolved the tensions within the Union over war aims” between conservative Democrats and anti-slavery activists because a language of deliverance “could serve so many ends” — it supported everything from conciliatory war measures to abolition.

The imprecise nature of deliverance allows Varon to fold parts of both the union and freedom arguments into her own. Freedom scholars are right that the North intended to remake the South, but union scholars are right that the North didn’t act from a desire to free slaves so much as a will to free the South’s white farmers and small planters from the tyranny of the slaveholding owners of vast plantations.

Varon’s argument is at times more novel than persuasive. Although she is a distinguished historian of antebellum politics, she rushes past the coming of the Civil War; the conflict is underway already in Chapter 1. This might be fitting in a work about wartime tactics but less so in a book about deeper motivations. The Civil War was an ideological conflict, developed over decades of painstaking political and intellectual fights that she largely skims past. Those conflicts shaped the concepts of deliverance, freedom and union. To understand the power of deliverance, we would need to see more about how the concept developed over time.

So, too, does Varon rush through her argument about the consequences of the Civil War. Deliverance may have fueled white Northern overconfidence in the efficacy of Reconstruction, and unconcern for freedpeople may have spawned apathy. But still, deliverance cannot explain the boldness and resilience of Republican support for civil and voting rights, nor can Republicans’ mixed motivations tell us much about the efficacy of their steps toward emancipation in 1861, abolition in 1865 and enfranchisement in 1867. A thorough reckoning with Reconstruction must engage with other issues: the fog of war, the idealistic vision of a self-perpetuating democracy, the resilience of local power, the weakness of the federal government. And above all that lies what seems the ultimate explanation for the disappointments of Reconstruction: an unbearably bloody white Southern counterrevolution. After all that is taken into account, it is not clear how much is left for notions of deliverance to explain.

While Varon doesn’t quite deliver on her argument about deliverance, she narrates battles and campaigns with an unusually deft, at times even gorgeous touch. This is some of the finest battle writing around, and a sweeping analysis of both United States and Confederate strategy and tactics. While the book can’t displace James M. McPherson’s “Battle Cry of Freedom,” still perhaps the single greatest volume ever written on the Civil War or even on United States history, it belongs beside it on the shelf. Given the volume of writing about the Civil War over the past 150 years, that is no small feat.

Armies of Deliverance

A New History of the Civil War

By Elizabeth R. Varon

513 pp. $34.95