Disunion follows the Civil War as it unfolded.

The summer of 1862 saw the Civil War’s two fronts as a mirror image. In the West, Federal troops had harnessed long stretches of the Cumberland, Tennessee and Mississippi Rivers to drive deep into the Confederacy. Fort Henry, Fort Donelson, Nashville and Memphis had all fallen, leaving Union forces largely in control of Kentucky, middle and western Tennessee, northern Mississippi and northern Alabama.

But in Virginia, Confederates were having a summer of unprecedented successes. Stonewall Jackson humiliated five different federal commanders in the Shenandoah Valley and at the Battle of Cedar Mountain. Robert E. Lee had stymied George B. McClellan’s Peninsula Campaign aimed at Richmond, and in August joined Jackson to humiliate John Pope at Second Manassas.

Confederate leaders saw this as the moment to capitalize on these successes with a bold military incursion into Kentucky in August. The Union’s breadbasket, the western border states lying astride the Ohio River, was about to become the next front.

Beginning in the mid-18th century, the Ohio River was one of the great highways of North America. Tens of thousands of people used it to float westward down from the Appalachian Mountains into the interior of the continent. The region filled up quickly: by 1790, 73,677 people lived in Kentucky, then a Virginia county, and 35,691 more in Tennessee. By 1810, 15 percent of the American population lived west of the Appalachians, by then including the newest state, Ohio. Within a decade, six more Western states would be added to the national Union. Three decades later, in 1840, more than a third of all Americans lived in this so-called First West.

The early settlements in the western region quickly thrived because of the river trade. The Ohio and its tributaries, which stretched north to nearly the Great Lakes, south to the Nashville Basin, and east to the Cumberland Plateau, sustained the growing population of the valley with crops and goods. Farmers loaded flatboats filled with products of their summer labors, with wheat milled into flour, corn distilled into whiskey and hogs slaughtered into bacon and soap. These and innumerable other goods floated down the Big Sandy, Scioto, Licking, Kentucky, Wabash, Cumberland and Tennessee, then down the Ohio to the Mississippi and on to even hungrier markets in Memphis, Natchez, New Orleans and beyond.

The appearance of the steamboat in the first decade of the 1800s revolutionized river traffic, making it possible to return upriver without walking, riding, pushing or pulling against the river currents. Before the arrival of the steamboat, items had to be carried over the Appalachians to western Pennsylvania and floated downriver. By 1820, 73 steamboats were working the Ohio and Mississippi rivers, bringing as much as 33,000 tons of goods back up to Louisville, Cincinnati and Pittsburgh. A canal-building craze soon followed, cheaply and efficiently connecting the inland areas, especially north of the Ohio, with the rivers. In 1852, at the peak of the steamboat trade, 8,000 landings were recorded at Cincinnati.

Owing in large part to the steamboat, in a single generation after the Revolution, bustling cities grew from these once isolated river towns: Pittsburgh, St. Louis, Louisville, Lexington, Cincinnati, Evansville. These were business towns — with regularity of design standard in all of these river cities, travelers talked of their attractive business climates rather than their physical beauty. Merchants dominated local society and politics, accumulating wealth by the southern river trade that both drove and exemplified their cities’ growing class stratification.

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But civic leaders also planted and cultivated the seeds of culture that sprouted first in these cities: newspapers, churches, opera houses and theaters, bookstores, museums, lyceums and debating societies, libraries and schools and colleges. Although St. Louis led all cities in the West in sophistication, by the early 19th century Lexington, Ky., was known as the “Athens of the West” because of its educational facilities, most notably Transylvania University. Other cities were not far behind. By the 1830s Cincinnati, the West’s Queen City with nearly 50,000 people, had replaced Lexington as the region’s cultural and commercial epicenter.

Fast on the heels of the steamboat boom came rail. By the eve of the Civil War, Ohio boasted some 3,000 miles of track, 76 times what it had in 1840 and the most of any state in the entire nation. Illinois was second in the region and nation with 2,799 miles, and Indiana followed (fifth in the country) with 2,163; only New York and Pennsylvania boasted more. Missouri and Kentucky, too, had engaged heavily in railroad construction, but their 817 and 534 miles of track, respectively, left them lagging far behind even most of the cotton states, much less their immediate neighbors. Even in Missouri, virtually all of the main railroad lines ran along or north of the Missouri River.

Industrial expansion in the West followed these states’ respective railroad booms, contributing to population explosions in all of them. The 1850s saw the floodtide. In 1860, Ohio’s 2.3 million residents represented a more than 50 percent increase since 1840, making it the third largest state in the country. Illinois’ population doubled each decade, reaching 1.8 million, while Indiana’s population had nearly doubled to 1.4 million. Kentucky’s population had, like Ohio’s, increased by half to some 919,000 residents and had spread noticeably westward.

Some of the new settlers came from Eastern cities, but many of them came from a new wave of immigration from Europe, which favored the railroaded portions of these states, creating new population centers away from the traditional riverine sections. As late as the 1840s, many of the unorganized areas or fledgling counties of the northern portions of the Northwestern states had been sparsely settled. But soon boggy forests were drained by industrious laborers and settlers, with railroads following, allowing these counties to account for much of their states’ growth in the final antebellum decade. All of the counties of northern Indiana and Illinois saw their populations double or more; even in Ohio, they increased by half in the decade. Propelled by the lake trade, Cleveland became a city and Chicago became the region’s colossus within decades of its founding.

The northward shift of these states’ populations contributed much to the western region’s population’s exceeding those of all other national regions, including the fast-growing cotton frontier. The towns and villages of the southern portions of these states, their traditional locus of population, declined proportionally. By 1860, between a quarter and a sixth of the population of the Ohio River states lay in the valley itself.

At the same time, though, the Ohio Valley cities thrived. Where in 1840 just less than four percent of the region’s overall population was urban (and only three cities boasted as many as 8,000 residents), by 1860 some 14 percent lived in villages, towns and cities, and the region boasted 14 cities with 10,000 or more residents.

This new urban population was unlike any the region had seen before. Between 1820 and 1849, nearly 2.5 million foreigners came to the United States, largely from northern Europe, representing a seven-fold increase in the incidence of immigration. By 1850, 47.2 percent of Cincinnatians were immigrants, and more than 70 percent came from either Ireland or Germany, in part responses to the terrible potato famine of the later 1840s in Ireland and the failed democratic revolutions of 1848 for the Germans. Nearly a contiguous square mile of the cityscape was a virtual German “stadt,” with bustling streets bearing names like Berlin, Schiller and Goethe, and with street signs and business placards posted in German script. The vibrant 10th Ward was known simply as “Über der Rhein,” or for Anglos, “Over-the-Rhine,” a descriptive term that originated from the area’s proximity to the Miami Canal, which separated it from the rest of the city.

Many, like those in Cincinnati, were Catholics, which for many native-born Protestants caused more consternation than the newcomers’ ethnicities. By 1850, Cincinnati was only the third most densely immigrant city in the nation. But those with more were also Western cities: Chicago and St. Louis. (Surprisingly, all three led New York City, in which foreigners constituted 45.7 percent of the population.) But immigrants flocked to all the Western cities. They made up some 17 percent of Louisville’s population. They likewise settled heavily in Covington, Ky., and Evansville, Ind., creating a unique culture. The diversity that became much of America was as much the western border region’s as the nation’s.

Many of these Germans were strongly antislavery.Yet as the second year of the war began, positions on slavery did not easily divide north and south of the Ohio. Many of the region’s Irish Catholics supported slavery’s protection, while in large sections of the free states fighting-age “Butternut” men, called “Copperheads” by their Republican opponents, laid out of the fight altogether or threatened to leave it, should they be conscripted or slavery be abolished by the “friction of war,” as Abraham Lincoln put it. Others sympathized outright with the Confederacy, and fights commonly broke out between them and their pro-war Republican neighbors.

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For many of the border region’s dissenting white residents, the course and events of the Civil War pointed out clearly that a new alliance had emerged, one in which Republicans in the Northern and Northwestern states appeared to be uniting in a conspiracy against liberty, which for many included the right of slaveholding. Angry and disillusioned, many of these Westerner dissenters sympathized with the region that now embodied their sense of betrayal and victimization — the beleaguered South. In the rural areas of Missouri, and western Kentucky and Tennessee, guerrillas were waging a desperate fight against occupying troops and local unionists that grew out of their recognition that the cities in their states were virtual fortresses: recruiting and staging centers for the Federal armies impossibly defended by hordes of blue-clad troops who guarded the supply and munitions depots and manufacturing centers for the federal government’s war machine.

As the newly appointed general in chief, Henry Halleck, later realized, Jefferson Davis and his advisers had “boldly determined to reoccupy Arkansas, Missouri, Tennessee, and Kentucky and, if possible, invade the states of Ohio, Indiana and Illinois while our attention was distracted by the invasion of Maryland.” Coordinated invasions of the border states on both sides of the Appalachians, as well as west of the Mississippi, would threaten the West’s major river cities and even the nation’s capital, perhaps turning the tide militarily.

But the decision was more than strategic; it was political. The federal government’s midterm elections loomed. Success on the new war front would embolden dissenters and moderates in the border states, especially in the Ohio Valley, to vote against Lincoln’s party, turn public support in the free states against the war, and possibly gain for the Confederacy its most elusive prize: foreign recognition. The cumulative effect would be to force the Lincoln administration to sue for peace. The Confederacy’s Tet Offensive was set to begin.

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Christopher Phillips is a professor of history at the University of Cincinnati. He is the author of six books on the Civil War era, including “Damned Yankee: The Life of Nathaniel Lyon” and the forthcoming “The Rivers Ran Backward: The Civil War on the Middle Border and the Making of American Regionalism.”