There was actually only one cause for the American Civil War: slavery. All the events leading to the Civil War, understood as steps moving steadily up the conflict, had slavery as the underlying cause for upset and increasing division between the North and South.

In 1935, the former Confederate captain, legislator, and historian Samuel A'Court Ashe (1840 to 1938) explained the reason for the Southern states' secession in his pamphlet A Southern View of the Invasion of the Southern States and the War of 1861 to 1865, writing:

Seven states seceded in the winter of 1860 to 61, and, on March 11, 1861, formed a new Confederacy of sovereign States with virtually the same Constitution as the United States. It created "a government proper," and the laws of Congress acted directly on individuals. The other Southern States seceded later when called on to engage in a war against this new Confederacy.

Why was that first secession? African slavery had existed in every colony and State and was particularly recognized and cared for in the Constitution, every State agreeing to return to the owner any fugitive slave. Without this recognition, there could have been no Union. An eminent justice of the United States Supreme Court, Henry Baldwin, of Pennsylvania, had declared slavery "the cornerstone" of the Government (Johnson vs. Tompkins, 1, Baldwin).

In time, the Northern States, whose shipping had brought many of the Negroes into the country, abandoned slavery. Still, every man who held office swore to support the Constitution. There was only one honest way out of the obligation to respect slavery, and that was to withdraw from the Union.

(45 to 46)

Samuel A'Court Ashe's work is not the only one to make clear that the preservation of the institution of slavery was at the heart of the Confederacy, the reason for secession, and the ultimate cause of the American Civil War. The Constitution of the Confederacy references slavery several times, as in Article 1, Section 4:

No bill of attainder, ex post facto law, or law denying or impairing the right of property in negro slaves shall be passed.

And in Article IV, Section 3:

No slave or other person held to service or labor in any State or Territory of the Confederate States, under the laws thereof, escaping or lawfully carried into another, shall, in consequence of any law or regulation therein, be discharged from such service or labor, but shall be delivered up on claim of the party to whom such slave belongs, or to whom such service or labor may be due.

These are only two citations, but there are many others in the document, making it clear that the institution of slavery was central to the Confederate States of America. Correspondence, editorials, and some, though not all, of the documents relating to the Ordinance of Secession make clear that the fear of the emancipation of the slaves by Northern states was the cause for secession, leading to the Civil War.

The events listed below, all having to do with slavery, bear this out as each one was another step leading to the eventual conflict that tore the nation apart between 1861 and 1865. In the words of the narrator of the excellent Kings & Generals video regarding the underlying cause of the war: "Spoiler Alert: It was slavery."

Slavery was becoming economically untenable before the invention of the cotton gin by Eli Whitney in 1793, which made cotton cultivation far more profitable than it had been and increased the need for slave labor to pick the cotton to be fed into the gins. Prior to the cotton gin, one sack of cotton could be produced for sale in a day; afterwards, 50 sacks or more could be made ready for sale in the same amount of time. The gin extracted the seeds from the cotton, eliminating that step in the process, and so all a planter needed was manual labor to pick the cotton. The more workers one had, the greater one's profit, and so the need for more and more slaves.

The end of the transatlantic slave trade in the USA in 1808 cut off the supply of slaves from outside the United States, although the demand for more slaves by Southern states was higher than ever. The abolition of the slave trade resulted in illegal smuggling operations in the South and attempts to stop them by the federal government. Illegal importation of slaves continued after 1808 up until 1860, when Clotilda, the last slave ship, brought the last slaves from Africa to North America. These operations contributed to further division between the North and South.

The Missouri Compromise admitted Missouri as a slave state and Maine as a free state, continuing the balancing act of the government to placate Northern and Southern interests regarding slavery. The fact that such a compromise needed to be struck illustrates how slavery had increasingly become a hot point in legislation between Northern and Southern states.

The push to abolish slavery in Colonial America was already underway by 1688 at the instigation of the German Quakers, but it gained notable support in the 1830s, especially after Nat Turner's Rebellion in 1831. The abolitionist William Lloyd Garrison championed the cause, although there were many working toward the same end of realizing the promise of liberty for all.

The deadliest slave uprising in US history, Turner's rebellion, in Southampton County, Virginia, encouraged widespread discussion of emancipation while also leading to harsher slave laws that were condemned by Northern abolitionists. Southern slaveholders blamed the abolitionists for the insurrection.

South Carolina challenged federal tariffs and asserted states' rights to nullify federal laws and regulations, establishing a foundation later used to justify secession.

Narrative of the Life of Frederick Douglass became the bestselling work dealing with slavery prior to the publication of Uncle Tom's Cabin by Harriet Beecher Stowe in 1852. Both works, as well as others by former slaves, increased the momentum of the abolitionist movement.

Harriet Beecher Stowe was a White abolitionist writer whose novel, depicting the difficult lives of slaves, became an international bestseller and was only bested in sales by the Bible.

Popularized by John O'Sullivan in 1845, this concept had been recognized since at least the early 18th century and, according to some scholars, informed the Declaration of Independence. Manifest Destiny was the belief that the United States was ordained by God to extend itself from coast to coast in forming a unified nation. This contributed to division between the states over whether Western Territories would be admitted to the Union as slave or free.

The Mexican-American War was informed by the concept of Manifest Destiny as President James L. Polk pursued measures to incite war with Mexico, which he was sure the USA would win. The Texas Revolution had separated Texas from Mexico, and Texas had become its own republic. It was annexed by the USA in 1846, entering the Union as a slave state, and setting off the Mexican-American War. When the USA won that war, the territories of the Mexican Cession (land won by the USA from Mexico) sparked controversy over whether they would be slave or free.

This legislation mandated that citizens of all states must aid in capturing and returning escaped slaves under threat of fine and/or imprisonment. The law was highly unpopular in the North, causing resentment toward Southern slaveholders. In the South, there was resentment toward the North due to the belief that Northerners were not honoring the act and were helping fugitive slaves escape to Canada. This legislation informed Article IV, Section 3 of the Confederate Constitution.

The Compromise of 1850 established the concept of popular sovereignty in territories, allowing the people to choose whether to become a slave or free state. The law admitted California as a free state but, at the same time, strengthened the Fugitive Slave Act.

This act allowed popular sovereignty to decide whether the territories of Kansas and Nebraska would be free or slave states, leading to the violence of "Bleeding Kansas" in which pro-slavery and free staters fought each other, often considered a "dress rehearsal for the Civil War." John Brown first gained notoriety during this conflict.

The controversial Dred Scott Decision ruled that Black people in America were not citizens, had no rights, and so could not bring lawsuits. It also ruled that Congress had no power to ban slavery in territories not yet part of the United States. The ruling enraged abolitionists, who then put greater efforts into the push for emancipation.

Abolitionist John Brown tried to incite a large-scale slave rebellion in Harpers Ferry, Virginia, in hopes of ending slavery in the United States. Brown's attack was widely understood by Southern slaveholders as expressing the intent of the North to abolish slavery by any means necessary. Brown became an archvillain to the slave-holding South but, after his execution, a martyr for liberty and equality in the North.

Ellen Craft was a light-skinned Black woman who posed as a White Southern gentleman in the company of her 'slave', her husband, William Craft, to escape slavery in Georgia and arrive as a free couple in Philadelphia, Pennsylvania, on Christmas Day 1848. After the Fugitive Slave Act of 1850 threatened their freedom, they fled the USA for England, where they wrote their book, Running a Thousand Miles for Freedom, which helped to further galvanize the abolitionist cause in the United States just before the Civil War.

Lincoln's platform of controlling the spread of slavery westward alarmed Southerners, who saw this as a threat to their way of life. The Republican Party's perceived anti-slavery platform was completely rejected by Southern slaveholders, who saw Lincoln as the chief representative of those views. After Lincoln was elected, South Carolina was the first state to secede from the Union.

The Secession Crisis is the name given to the secession of the states that would come to form the Confederate States of America under their president, Jefferson Davis. The Ordinance of Secession is the term for the resolutions by the states separating themselves from the United States between December 1860 and February 1861. The states that would eventually form the Confederate States of America were:

The last four – Virginia, Arkansas, Tennessee, and North Carolina – voted for secession after President Lincoln called for a mobilization of troops following the Confederate attack on Fort Sumter on 12 April 1861, which started the Civil War. These states (as well as the others) did not believe that Lincoln had the right to use military force to preserve the Union, opting instead to hold that any state whose interests were no longer valued by the federal government had the right to separate themselves and form their own government with laws that would uphold the institution of slavery.

These eleven states would fight for the right to preserve the "peculiar institution" of slavery in the United States between 1861 and 1865 and, afterwards, would claim that they were waging war for the noble cause of "states' rights"– a false claim still repeated today and believed by many in the United States and elsewhere.

According to this argument, the states that formed the Confederacy had a legal right to secede when they felt their rights were no longer being respected or represented by the federal government. Secession, according to this claim, had nothing to do with slavery but was an honorable course chosen in the cause of liberty and autonomy of individual states – in the same way that the original Thirteen Colonies had revolted and broken away from Great Britain during the American Revolution and American Revolutionary War.

But this claim is demonstrably false – as evidenced by A'Court Ashe's words above as well as many others. Initially, Lincoln had no interest in freeing the slaves, only in preventing the spread of slavery to Western Territories that would become states and, after secession, in preserving the Union. The Civil War was fought by the Confederate States of America to preserve the institution of slavery and by the forces of the North, after 1863, to preserve a Union of states free from the fundamental betrayal of the concept that all people are created equal.