Last week's tragic shooting spree in Charleston has prompted a movement to remove symbols of the Confederacy from public buildings across the South. And that has prompted some people to deny that the Civil War was about slavery.
Obviously, any conflict has multiple causes. But the evidence is overwhelming that slavery was the primary issue that drove the South to secede from the Union. One way to see this is to look at the geography of secession.
This map shows the percentage of local populations that were slaves in 1860:
Slavery was most common in areas where conditions were suitable for large-scale cotton plantations. These areas included southern Alabama, much of Georgia, and South Carolina.
Other areas, shown in yellow in the map, had a lot fewer slaves. For example, the mountainous regions of West Virginia and eastern Tennessee weren't suitable for large cotton plantations, so they had fewer slaves. Slavery was also less common in northern Alabama than farther south.
Kentucky and Missouri had a lot fewer slaves than states farther south. They decided to remain in the Union.
Now look at this map showing how counties in the South voted on secession at the beginning of the Civil War:
Comparing the two maps shows a pretty clear pattern. The regions that voted against secession were generally those that had the fewest slaves. Anti-secession sentiment was so strong in West Virginia that people there formed their own state that stayed in the Union. Eastern Tennessee was a hotbed of Unionist sympathies. In northern Alabama, there were "yeoman farmers mixed with large slaveholders in the river bottoms and fertile plains," producing a population that was split between Unionist and Confederate sympathies.
The parts of the South that relied most heavily on slave labor were the ones that were most eager to secede. The Civil War was about slavery.