Was slavery the cause of the American Civil War?

Yes and no.
Slavery was the defining issue of America’s first eighty years. It was the foundation upon which sectional differences between North and South were established. And, slavery was the impetus, the engine behind all other issues that worked together to drive America to Civil War.
Slavery was indeed the cause of differences and conflict between North and South; but it alone did not hurtle America into open war and bloodshed.
Shortly after Eli Whitney’s patenting of the cotton gin, slavery became known North to South as that “peculiar institution” upon which the United States economy, North to South, depended.
As despicable as the institution came to be in the American North, slavery was still largely tolerated where it currently existed. Many Northerners were willing to leave the institution alone in the South, knowing perhaps what the repercussions of summarily ending involuntary labor would mean to the economy in the North, where most textile mills and factories processing cotton and tobacco were located. Northerners were also sensitive to the impact that slave-picked cotton had on international markets.
As such, there was a general angst among Northerners about abolishing slavery altogether, and immediately, as many abolitionists demanded.
The American North, however, was overwhelmingly united in its opposition to the expansion of slavery into the new territories west of the Mississippi River. While Americans in the North, by and large, tolerated the existence of slavery in the Old South, they were not prepared to tolerate its perpetuation into newly acquired territories, where it would exist in direct (and unfair) competition with free labor.
Northerners generally felt that a peaceful end to slavery was not only possible, but probable, if limited to the Old South. The institution would be gradually phased out by both agricultural and economic conditions.
But the North’s insistence on restricting slavery in the new West only antagonized the South, which perceived a Yankee conspiracy to destroy the Southern economy and culture.
Tensions over slavery really began to tighten in 1820 when Congress enacted the Missouri Compromise, a law that prohibited slavery in the territories above the 36-30 parallel and allowed Missouri to enter the Union as a slave state.
Things only got worse in 1846 when Mexico declared war against the United States over Texas, which the latter had annexed as a state in late 1845. Antislavery factions in the North accused the federal government of going to war to protect Southern slave-holding interests, because Texas was being admitted as a slave state. Meanwhile, Southern pro-slavery interests wanted Texas’ representation in Congress.
Southern statesmen like Sen. John C. Calhoun of South Carolina argued that if Southern interests did not expand into the new territories, representation in Congress would remain indefinitely in favor of Northern interests. The North, Calhoun pointed out, held the lion’s share of population and, therefore, had the most Congressional districts in the House of Representatives. The South’s only hope for fair and equitable representation, Calhoun reasoned, rested in the Senate. As such, every new territory seeking statehood represented an opportunity for Southern interests to increase their numbers—and their influence—in Congress. Thus, this was the heart and core of the state’s rights argument. But that is a topic for further discussion elsewhere.
The point is that the Mexican-American War was a contentious issue between Northern antislavery and Southern pro-slavery movements because of the prospect of expansion.
The acquisition of former Mexican territories—California, in particular—further broadened the divide between northern and southern interests as settlers from both regions emigrated into the new territories and vied to establish them as either slave or free. The Compromise of 1850 was a temporarily effective measure meant to appease both sides and avoid escalating the sectional conflict into bloodshed. In effect, it banned slavery in parts of the newly annexed Mexican territories—namely California—while allowing by vote of popular sovereignty in others, such as New Mexico and Utah. The Compromise also created a stronger, more expansive Fugitive Slave Act, which satisfied the South, but enraged the North.
As such, sectional conflict was merely avoided for the time being, while tensions between the two regions only continued to grow and relations worsened.
But no other westward expansion became as volatile or did as much to accelerate America closer to bloodshed than the settlement of the Kansas and Nebraska territories in the 1850s.
At issue was the 36-30 north parallel designated by the Missouri Compromise of 1820. Pro-slavery interests contended that this law was unconstitutional, because it banned the owning of slave property, which was argued to be unconstitutional per the Fourth and Fifth Amendments to the U.S. Constitution.
The territories of Kansas and Nebraska were split by the line between slave and free. The Kansas-Nebraska Act of 1854, proposed by Democratic Sen. Stephen Douglas of Illinois, essentially repealed the provisions of the Missouri Compromise, allowing popular sovereignty to decide the issue of slavery in the Kansas and Nebraska territories.
Unfortunately, the act resulted in far-reaching repercussions and tragic consequences. Political shenanigans and bully tactics of pro-South border ruffians from Missouri enraged free-soil (antislavery) settlers, some of whom responded with bloodshed.
John Brown was the most infamous example of violence in what became known as “Bleeding Kansas.” His active, leading role in committing the cold-blooded murders of the Pottawatomie Massacre in 1856 was a microcosm of the level that sectional conflict had reached in America, and offered a grim glimpse into the future of what lay ahead for the nation.
Abolitionists in the North conveniently spun the Massacre as killing in self-defense, thus justifying the deaths of the pro-Southern settlers. Both the Massacre and abolitionist societies’ defense of it enraged the South.
Both sides ultimately felt violated by the Kansas-Nebraska Act: The North, because the Missouri Compromise was repealed and because popular sovereignty left the unsettled issue of slavery open to all sorts of political tom foolery. And the South, because the same popular sovereignty it had supported ended up fueling the hatred of murderers like John Brown, who took out his wrath on pro-Southern settlers.
Brown would take his antislavery crusade into Virginia three years later in a maniacal plot to arm the slaves and lead a massive, violent revolt across the entire South. His near success at seizing the federal arsenal at Harper’s Ferry, Virginia, caused high alarm among many Southerners, whose foremost fear was not Yankee (Northern) invasion, but unrest and revolt among the millions of slaves in the South.
The South was no stranger to slave uprisings. Several took place in the early Nineteenth Century. Some were thwarted before any harm could come from them, while others, like the rebellion led by slave Nat Turner in Virginia, resulted in the murders of dozens of Southern whites.
The very last thing the South was going to stand for was Northern support for a slave uprising. It was one thing for the North to oppose the South’s peculiar institution, but quite another to actually encourage a rebellion among the slave population.
Hesitation among Northerners to condemn John Brown for wanting to end slavery convinced the South that this wasn’t going to be the last time that a slave uprising would be enabled by the North. As such, secession talk started up yet again, but this time more heated, more passionate, more insistent, and more defiant than ever before.
In the North, slavery abolition also became more passionate—and more important to the national debate—than it had ever been before.
The stage was set not for an inevitable war, but an imminent one. America had passed a point of no return with the John Brown-Harper’s Ferry incident, which was really a residual extension of the “Bleeding Kansas” conflict.
It was the last straw for both the North and the South. From that point on, all other attempts at compromise—including the Crittenden Compromise in December 1860—were futile, at best. They no longer delayed conflict, and served only to demonstrate how far gone off the edge of reason America had fallen.
Slavery, without a doubt, was the driving force that led to the American Civil War. But was it really the cause of war? Was slavery the “causus belli” issue that drove millions of Americans to take up arms against one another?
History suggests, and evidence supports, that it was not.
The Merriam-Webster Dictionary defines “cause” (noun) as a motive, or the reason for an action or condition, while “motive” is defined as something that causes a person to act.
History shows that slavery alone was not motive enough to cause the North to act against the South, or for the South to act against the North.
As stated earlier, many Northerners, while detesting the institution of slavery, were willing to let it exist in the Old South if it meant tempering the sectional conflict and averting violent confrontation. What Northerners opposed, though, was the expansion of slavery into the new territories west of the Mississippi River and north of the 36-30 parallel, as well as slavery’s unencumbered invasion of the free North via the federal Fugitive Slave Act.
Although slavery was the impetus of conflict between North and South, expansion appeared to be the catalyst of tensions that accelerated and later hurtled the nation imminently toward formal, open war.
While expansion could not have sparked war without slavery, likewise slavery could not have caused war without expansion.