It's useful to describe what it means to say slavery was the root cause of the war.
In this video, historian Elizabeth Varon talks about the "fundamentalist school" in the first 3-4 minutes of a talk. She says
"there's emerged in recent years a strong consensus, which scholars call the fundamentalist school, that slavery was the root fundamental cause of the civil war and
that the political antagonisms between the North and South flowed from the fact that the North was a free labor society while the South was a slave labor society which remained committed to slavery and indeed to extending its domain."
When historians say that slavery was the root cause of the war they mean to say the following:
• At its root, the war was caused by the conflict between the free labor North and the slave labor South. That is, "slavery" did not cause the war, per se; rather the
conflict between the sections over free labor versus slave labor was the root cause of the war. This conflict led to political antagonism that resulted in war. In this conflict, countless political, social, and cultural battles became proxy wars that became a full-fledged military war by the 1860s.
• The conflict of free labor versus slave labor was the
primary and
essential element in causing the war. That is, it is difficult to conceive there being a Civil War without this element. There were other sectional conflicts, but they did not rise to the level where secession and ultimately war were seen as necessary to resolve the conflict.
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To add to what historians mean when they say the conflict between free labor and slave labor was the root cause of the war, there is this. In his book
At the Precipice: Americans North and South during the Secession Crisis, author Shearer Davis Bowman writes
The inability to engineer an acceptable compromise during the secession crisis reflected the reality that neither northern nor southern stalwarts could endorse concessions that did not seem to undermine what they perceived to be their fundamental interests rights and honor as citizens of the American Republic.
Avery Craven has powerfully concluded, "neither the North nor the South could you yield its position because slavery had come to symbolize values in each of their socio-economic structures for which men fight and die but which they do not give up or compromise."
That last comment is key:
the conflict of free labor versus slave labor was uncompromising. There was no middle ground between free labor and slave labor. Or at least, the antagonists could not find one. Other elements, such as tariffs, could find a middle ground for resolution. The North and South could not find a middle ground on the fundamental conflict.
I would add on this point that studies of intellectual history, emotional history and rhetoric indicate that the sections had evolved hardened views of their labor systems into which so much emotional, intellectual, political, social, and political capital had been invested, that they could not give up on their views. For the sections, free labor and slave labor were not just theories or world views; these things represented
who they were culturally, intellectually, socially, and so on. These were not just conflicting world views, they were conflicting identities. These identities were entrenched to a point that compromise meant, as Shearer Bowman put it, giving up on their interests, rights, and honor. The sides were not going to do that, and then the war came.
- Alan