What you’re asking about is commonly referred to as “The Lost Cause”, a pseudo-historical movement that spawned soon after the end of the Civil War, followed by bursts of activity at the turn of the 20th century and in the 1950s and 60s. And, unfortunately, it continues today.

Specifically with regards to your question, the unfortunate answer is that while it may seem incredibly obvious to us that the Civil War was based on slavery, the Lost Cause is built on bad faith arguments, logical fallacies, and incorrect conclusions drawn from admittedly true facts. So, while we can point to the Confederate Constitution as clear evidence of their commitment to preserving slavery, the Lost Cause is averse to dealing with the realities of history and frequently twists the facts of the Civil War to defend their own agenda.

From a historian’s perspective, the Lost Cause can be infuriating to debunk because oftentimes, the initial statements are technically true but often lack important information or fail to lead to the correct conclusion, either by the omitting of crucial facts or willful ignorance.

In my experience, the Lost Cause commonly cites two pieces of information as “evidence” for their claims that the Confederacy was morally correct and the Union was in the wrong, and I’ll address each one separately.

1. Confederate soldiers were largely not slave owners themselves and were fighting to defend their homeland.
2. The North only cared about ending slavery for political and economic gain.

The first of these claims is somewhat true, and here we see the inherent problem with pseudo-history. By presenting true statements and twisting the conclusions, pseudo-historians are able to legitimize their arguments to some extent, since it takes more critical thought to dissect an argument logically than to simply read conflicting statements. Based on a study of volunteers in the army of Northern Virginia in 1861, historian Joseph T. Glatthaar estimates that 1 in every 10 Confederate soldiers owned slaves themselves, and 1 in every 4 lived in a slave-owning household. While these numbers are higher than the general population of the South, which clocked in at 1 in 20 and 1 in 5, respectively (Glatthaar, General Lee’s Army), this is a prime example of a true statement being twisted into a lie.

“Ninety percent of Confederates never owned a slave,” cries the Lost Cause, “and seventy-five percent didn’t come from slave-owning households,” they continue. “Clearly, these men, and by extension the Confederacy, weren’t fighting to preserve slavery!” The problem with this claim is that it both disregards the societal benefits of slavery, even to the non-slave-owners themselves, and conflates the ideologies of the foot soldiers with the ideologies of the government.

First, one did not have to be a slave owner to benefit from slavery; as Eric Foner points out, there is a difference between a slave owning society and a society that owns slaves, and the Antebellum South was very much the former (Foner, America’s Unfinished Revolution). If one lived in the Antebellum South, one benefitted from slavery in myriad ways, from the growth of the economy to lower prices on local goods as a result of the unpaid labor of slaves. So the idea that a Confederate soldier who did not own slaves would not have been supportive of slavery is bunk. And, to be perfectly blunt, many were just outright racist, unwilling to think of their slaves as people and often willing to congratulate themselves and their families for being kind, humane slave owners who fed, clothed, and sheltered these inferior beings while educating them about the Christian God.

What is true, however, is that while Confederate soldiers as a collective benefitted from and supported the institution of slavery, relatively few picked up their rifles and charged into battle thinking “I’m doing this to preserve slavery”. Your common, everyday man on the ground in a grey uniform typically believed himself to be a defender of his homeland, his state, his territory; that much is true. HOWEVER. We absolutely cannot make the mistake of conflating the thoughts of a soldier with the thoughts of their government. The Confederate government was founded on, and fought for, the desire to preserve slavery against the wishes of the federal government, and to suggest otherwise is blatantly false and anti-historical. What a solider thinks and what his government thinks can be two entirely different, and even contradictory, things.

The second of these claims, which argues that the North only cared about political and economic gain, not the destruction of slavery, is similarly partially true but not fully indicative of the bigger picture. Yes, there were many people in the North, and even in the Northern army, who fought the Civil War because they had political and economic interests that would have been disrupted had the Southern states seceded. It would be irresponsible to claim otherwise, or to say that every Union man was fighting for the freedom of the enslaved people in the South. Yes, some Northerners owned slaves as well. Yes, some Northerners believed that blacks were inferior to whites, that the races could not co-exist, and that the black communities should be shipped back to Africa. No, the North was not a utopia of racial equality.

One of the most famous quotes regarding this topic comes from an 1862 letter Abraham Lincoln wrote to newspaper editor and abolitionist Horace Greeley, when the President wrote:

I would save the Union. I would save it the shortest way under the Constitution. The sooner the national authority can be restored; the nearer the Union will be "the Union as it was." If there be those who would not save the Union, unless they could at the same time save slavery, I do not agree with them. If there be those who would not save the Union unless they could at the same time destroy slavery, I do not agree with them. My paramount object in this struggle is to save the Union, and is not either to save or to destroy slavery. If I could save the Union without freeing any slave I would do it, and if I could save it by freeing all the slaves I would do it; and if I could save it by freeing some and leaving others alone I would also do that. What I do about slavery, and the colored race, I do because I believe it helps to save the Union; and what I forbear, I forbear because I do not believe it would help to save the Union.

This letter is frequently trotted out by Lost Causers as “proof” that Lincoln was not fighting the war to abolish slavery, and was, in fact, merely fighting to preserve the Union. This, it is argued, lays bare the hypocrisy of the Union and proves that the War of Northern Aggression was a political move by a desperate man seeking to hold onto his precious power, slaves be damned.

However, while this letter may appear to be so on the surface, three important pieces of information are missing.

One, Lincoln was not the only person involved in the Union war effort, and thousands of people in the North had been clamoring and fighting for the abolition of slavery for decades (more on them in a minute).

Two, Lincoln closed his letter by reminding Greeley that “I have here stated my purpose according to my view of official duty; and I intend no modification of my oft-expressed personal wish that all men every where could be free.”

Three, in this time period, government and military service were based much more strongly on the idea that one should lay aside their personal desires and feelings in service to their country, which was the responsibility of every able-bodied American man. After his nomination for President, Ulysses Grant wrote to his friend William Sherman that “[the position of President] is one I would not occupy for any mere personal consideration, but…I have been forced into it in spite of myself” (Brands, The Man Who Saved the Union).

While plenty of politicians were happy to serve their own agendas, many others dictated themselves and their policies according to a principle of asking what would be best for the country and people they had been called to serve. Here, we see the dichotomy between Lincoln the President and Lincoln the man. As a President, Lincoln felt that it was his responsibility to protect the country he had been elected to care for, and the preservation of the United States at all costs was paramount; as a man, Lincoln had abhorred slavery for years and would have been happy to see it eradicated from the face of the Earth.

Circling back to the first of these points, we cannot discredit the thousands of white and black abolitionists of the North who dedicated their lives to the utter destruction of slavery at no benefit, and often great cost, to themselves. Politicians like Charles Sumner and Salmon Chase, soldiers like Ulysses Grant and William Sherman, journalists like William Lloyd Garrison and Horace Greeley, and a host of people like John Brown, Frederick Douglass, Harriet Tubman, and Henry Ward Beecher; all of these people and countless more argued for, advocated for, and in some cases, fought to the death for the abolition of slavery. Many of these people had no economic or political reasons to oppose slavery, they fought for what they believed was morally right and no reason besides.

Even a cursory reading of history lays bare the truth that the Civil War was based on nearly a century of political infighting over slavery and its effects on the country. Everything the Confederacy did from secession to Appomattox was done to preserve slavery, the bedrock of Southern society and the foundation of all Southern politics from the Revolution onwards. More in child comment.

Edit: clarification.

While the loudest and most vociferous anti-Union Southerners loved to say that the North would stop at nothing to abolish slavery, most serious Southern intellectuals and pro-secession politicians were not immediately worried about such direct action. Instead, they feared that one of the stated goals of anti-slavery activists and abolitionists would succeed: that of ending the expansion of slavery in the United States territories.

Southerners truly believed that slavery needed to constantly expand or else it would collapse. Their argument was that slave populations would consistently overtake the white population of slave states, leaving them vulnerable to slave insurrections. This was itself a product of the Haitian Revolution, leaving Southerners convinced that they needed the territories as an outlet valve to prevent a violent uprising hell-bent on killing all the whites. Uprisings like the German Coast Revolt, Nat Turner Revolt, and John Brown's Raid on Harper's Ferry further reinforced these ideas.

So pro-secession politicians could argue to a believing Southern public that their very lives depended on the continual of slavery. They managed to win a number of political victories in the 1850s, starting with the new Fugitive Slave Act and culminating in Dred Scott v. Sanford. However, this also produced a backlash that propelled the Republican Party (an explicitly ant-slavery party) and their candidate, Abe Lincoln to the presidency. This signalled to many that the South could no longer guarantee the expansion of slavery, and thus the preservation of slavery itself, while a part of the Union. They were likely mistaken, as it was the war itself that gave Lincoln the power to enact an anti-slavery agenda. Lincoln likely would not have been as successful had it not been for the war.

Edit - typed 1950s instead of 1850s, succession instead of secession

Source:

Matthew Karp, This Vast Southern Empire: Slaveholders at the Helm of American Foreign Policy

Aaron Sheehan-Dean, Why Confederates Fought: Family and Nation in Civil War Virginia