We were rereading David Potter’s timeless book The Impending Crisis: 1848-1861 and thinking about how it sounds to us today to read that some Americans valued union over abolition.

It sounds awful. It sounds like cavilling, cowardice, inhumanity, and loathsome empty-headed pseudo-patriotism.

But that is because this is not 1850, or 1858, or any point antebellum. It’s so hard to forget or to avoid the impact of hindsight on historical vision. We know that the Civil War did not end in permanent disunion, that the United States continued, grew, and thrived after 1865. But as Americans before 1861 contemplated the possibility that slavery might cause a civil war, they did not know any of this. They only knew that a civil war might erupt over slavery.

Think of it this way: what if right now, as you sit reading this, the United States was in danger of civil war. Some group of states had actually written up papers outlining how they would secede, and they had the power and the foreign backing to do it. Imagine that every week you read about how these states—let’s say 15 western states—were ready to actually sever their ties to the U.S., and leave the nation with 35 states and a big hole.

It’s impossible for us to really imagine this. We are faced daily with serious threats to our economic, intellectual, and political unity—there’s constant talk about red and blue states and how the coasts hate the  middle and vice-versa, etc.—but we cannot imagine this translates into a threat to our actual political unity. We can’t picture facing the possibility that civil war would break out over these issues and that the United States as we know it would cease to exist.

And all over one political and social issue. An important issue, to be sure, but not one that you thought could destroy the United States. Say it was illegal immigration. It’s been simmering for decades, but it’s begun to boil in the past 10 years, and people’s emotions are getting stronger about it. What do you think will happen in this situation?

Well, you expect it to keep dragging along as a divisive issue that will someday get enough minor legislation to die down, and be replaced by something else. Inertia or a solution, those are the options.

You never expect it to cause an actual civil war, with people in your state fighting people from another state. You don’t expect to see armies formed in the western U.S. states to fight the U.S. amed forces. You don’t expect to have your home destroyed by battle next year.

And that’s the way Americans viewed slavery in the antebellum years. It was a divisive issue, and was getting hotter after 1848, but civil war? Really?

Once it became frighteningly clear that the southern states really were prepared to secede, and really were gathering an army to fight the rest of the U.S., many Americans clung to union as a sacred obligation in order to forestall war. And you can see why, if this thought experiment has worked. If our nation were about to actually go to war over illegal immigration right now, wouldn’t you protest that union was more important than this issue? that the issue could surely be worked out legislatively instead?

Of course, illegal immigration is not truly like slavery, but the conditions many illegal immigrants work and live in here in the U.S. can approach slavery. And the hatred and revulsion some Americans feel for these people is equal to the hatred and revulsion some Americans had for enslaved black Americans. And, like free black Americans, even legal immigrants are in many cases mistreated and denied equal access to the law. Still, could we possibly go to civil war over it?

Americans felt the same way in the antebellum period: yes, slavery was a big issue, but it was just one of a half-dozen big issues. Why go to war over slavery? It seemed irrational. It seemed much less of a sacred cause than union, the great union of states that shone the light of democracy over the world.

This is just an attempt to really put ourselves in the shoes of our ancestors. When we do so, we can better understand why the cry of “union!” was stronger for many Americans than the cries of secession or abolition. Only when we truly try to put ourselves in those shoes can we begin to understand the seemingly incomprehensible decision to support union over abolition that many of our ancestors made.