By 1860, slavery had become such a divisive issue between the North and South that the two regions of America literally went to war over the issue.

But it wasn't always this way. As this map (and its interactive version) by historian Lincoln Mullen at George Mason University shows, slavery wasn't that rare in the North shortly after the US's birth, although it would be completely abolished over the next few decades:

The map is haunting, showing the rapid spread of a, needless to say, deeply abhorrent practice. But here are a few things that stuck out to me:

It took a few decades for all Northern states to abolish slavery. But by 1860, a clear divide between the North and South remained when it came to slavery.

A few states — Maine, Massachusetts, Vermont — consistently had few to no slaves.

Slavery spread quite quickly. As it concentrated in the South, regions that previously had few or no slaves quickly became majority-slave areas.

There were a lot of slaves in the South. In several counties, slaves made up 90 percent or more of the population. As Mullen put it, "When looking at the density of the population that was free, large swathes of the South appear virtually depopulated."

Beyond the historical points, the map shows something that still affects modern America: Some researchers have found, for example, that the size of Southern counties' slave populations in 1860 can predict some people's political beliefs today.

Source: Lincoln Mullen, "The Spread of U.S. Slavery, 1790–1860," interactive map, http://lincolnmullen.com/projects/slavery/, doi: 10.5281/zenodo.9825.

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