It's complicated. Saying that abolitionists caused the Civil War used to be a thing historians did regularly, back when the abolitionists were generally reviled as a bunch of religious fanatics. It's fallen greatly out of favor, through there are occasional vestiges of it in works of recent decades. But it's also fair to say that without abolitionists, there would have been no Civil War because there would probably have been no threat to slavery which then justified secession once the nation elected an antislavery president. So let's untangle this.

For the entire antebellum, abolitionists are a tiny minority of white opinion and often openly reviled in the North. Mobs attacked abolitionist meetings, destroyed abolitionist papers, and they were often treated as basically the scum of the earth. Until around 1830, the movement is so small among whites that it's hard to even speak of it as a movement. Most white opinion in the North, when opposed to slavery at all, is largely opposed to the expansion of slavery rather than oriented toward ending it in general. Historians usually call this larger group antislavery, and the smaller attack-slavery-where-it-is group abolitionists. All abolitionists are antislavery, but few antislavery people are abolitionists.

The two positions exist on a continuum, of course. On the antislavery end people are more interested in working within the existing constitutional and political framework. They vote in elections, petition representatives, and even join national political parties that put them in coalition with slaveholders. They tend to believe that the Constitution is at least potentially proslavery and that preserving the Union is a way to erode or at least sideline the institution.

The first generation of white abolitionists largely come up in that tradition, which was broadly credible. It banned slavery from the Northwest Territory. It had emancipated slaves in the North, taking the institution out (albeit gradually) from every Northern state ending with New Jersey in 1804. That success was followed up by outlawing the import of slaves from abroad on the first instant that it was constitutionally permitted, the first day of 1808. That vote came, incidentally, with all of five dissenters in the Congress.

To be sure, it's not all roses and parades. A form of slavery in all but name is instituted in Illinois. Slaveholders remain the dominant power at the national level. Slavery expands into the modern Deep South, where it thrives in the cotton boom. But it's clear that at least until the Missouri Crisis, the existing political order can deliver antislavery victories of a kind.

The Missouri Compromise changes all of that because it's not a compromise in any meaningful sense. At issue was the admission of Missouri with slavery, or with slavery set on a road to extinction. The antislavery side wanted to make Missouri's admission to the Union contingent on the latter point. They lost. The famous line drawn through the rest of the Louisiana Purchase to exclude slavery elsewhere was only a promise that potentially future states might not have slavery, whereas Missouri got to keep slavery then and there, in perpetuity. It was actually drawn largely to give northerners inclined to vote for slavery in Missouri cover for voting against the popular passions of their section. You can tell by how the vote went, with a majority of the South (51-45, combined Senate and House vote) in favor of the line.

One can argue that it was close, but here's how the section voted on banning the introduction of additional slaves to Missouri: 1-81 against. And the actual program of gradual emancipation floated for the state: 2-81. In the context of those margins, it's quite clear that when an actual material sacrifice was on the line, the South came out almost unanimously for slavery.

From an antislavery POV, the things they wanted from Missouri were hardly radical. They didn't want to emancipate Virginia or South Carolina, but rather a jurisdiction with ten thousand or so slaves. That's not far from what New York had when it embarked on its plan of emancipation in 1799, an especially pertinent example given that the man who introduced the slavery restrictions was James Tallmadge, Jr. of New York.

So the antislavery side asked little and got nothing out of the so-called Missouri Compromise. This led to about a decade of disillusionment that for some coalesces in the conviction that the existing order cannot be counted on to end slavery, now or in the future. The most famous exponent of that viewpoint is William Lloyd Garrison, who eventually decided the Constitution was a proslavery document and the Union fundamentally a slavery-preservation machine. (The recent trend among historians, myself included, is to agree with him there.) So the strategy becomes to press for immediate, unconditional abolition everywhere...but also largely by means of moral suasion and fiery rhetoric rather than by working within the system which had betrayed them.

We know how well that strategy worked out.

Not everyone followed Garrison out of practical politics, but the 1830s are a low period for antislavery fortunes. The developing Second Party System is built in part around keeping enough of them within the parties and marginalized so as to render antislavery impotent. If they quit the national parties, as Garrison had, but remained active in conventional politics of some kind, then they might for an avowed antislavery party which would unite the North and unite the South against it.

That's what happened, but it takes a long time. Essentially a series of southern advances makes part of the Garrisonian message more plausible to many in the white North, each one moving the needle a little bit, from which it then slips back not quite to where it was before. There are many of these, large and small. I can expand on them if you like, but for right now I'm going to do a quick overview of the biggies.

The Gag Rule, passed by the House of Representatives, denies reception to antislavery petitions. This is an outrage and an affront, because petitioning is the main way that voters communicate with their representatives. It was customary to receive any petition sent and reps would present them to the House even if they rejected the suggestions inside. There are multiple gag rules over a period of some years, which of course generate still more petitions and a group of northern congressmen led by John Quincy Adams (who has his finest moments outside of the Amistad case in all this) work overtime to find ways to get them on the House floor all the same. JQA basically trolls southerners with this, demonstrating that he can lawyer he way around just about any rule they put down and forcing them to decide between the First Amendment and slavery. They went for slavery. It's a long-running controversy that helps keep slavery in the news and make northerners mindful of just how highly southerners regard their rights.

The annexation of Texas. The accession of a large, slaveholding jurisdiction with unclear boundaries and unrecognized independence to the Union was too hot for Martin Van Buren to handle, but John Tyler was rejected by his party and saw it as a way he could revive his political career. Maybe he could even get elected president instead of inheriting the job. The Texans are interested, Tyler is interested, the Democrats are interested. Most Whigs (Tyler's former party) are not. It's enough that they can't get the 2/3 required for a treaty, so instead annexation takes place by a joint resolution of Congress. This was at the time viewed as the height of dubious maneuvering and kicks off the next big battle.

Texas disputed border with Mexico gets disputed by James K. Polk, the guy who replaces Tyler. He provokes the Mexican War, which the US wins and claims the modern Southwest as its prize. Mexican law banned slavery there. Would American law follow? This isn't the Louisiana Purchase, but rather territory largely unspoken for. David Wilmot (D-PA) proposes that any land taken from Mexico (except Texas, which he called a mulligan on) should have a slavery ban just old the Northwest Territory once had. Cue a tremendous, multiyear firestorm that makes the Gag Rule stuff look like a tea party.

The eventual resolution of all that makes things almost completely worse. The antislavery side gets a one thing that the proslavery side probably couldn't have denied them with any amount of trying (a free California, the Californians insisted) and a ban on the slave trade in Washington. That doesn't mean no slavery in DC, only that if you want to hold an auction for people you need to go over into Maryland or across the Potomac into Virginia. This is something that antislavery petitioners had asked all the way back to the gag days, but it's also about the least Congress could possibly have done. In exchange, Utah and New Mexico are open to slavery and the North has to deal with a draconian fugitive slave law that criminalizes the Underground Railroad and conscripts any random northerner who happens to be about into slave catching on penalty of fines and imprisonment.