The only cause at stake in the Civil War was the Southern states' desire to perpetuate slavery. Southern soldiers knew that because many of their states set out the reasons for secession in various resolutions and declarations in the attempt to justify their ordinances of secession. There were no Confederate heroes; each and every Confederate fought to maintain human bondage. No Confederate flag stands for anything other than the atrocity of chattel slavery.
It seems odd that it is necessary to make this point in the 21st century, but generations of Southern school children have been taught the "Revisionist" history, which was more or less consciously developed and inserted in Southern curricula, not only so little white children could avoid feeling badly about the wicked acts of their evil ancestors, but also to support the continued practices of segregation and racism. In every capital in South, even today, it is easy to find monuments celebrating the defense of slavery, and in many cases one need only look at the state's flag to see the continuing denial of the South's guilt.
Everywhere else in the world free people demolish (or at least remove from plain sight) the statues and monuments of tyrants, but in the American South, they build new ones. Stalin goes down, Hitler goes down, Saddam Hussein goes down. But there is always a new statue of Robert E. Lee or a named or unnamed Confederate soldier, for the celebration of tyranny is a way of life in the South.
Poor students of the bad revisionist history today tend only to remember "states' rights" as one of the revisionist causes. There were a few others, such as tariffs and the tit-for-tat admission of new states as free and slave states. But as the Southern states' resolutions make clear, these complaints were seen only as attacks on slavery. There is no conceivable way, of course, that any state ever had the right to make one human being the property of another, but by the time the Revisionists were writing Southern textbooks, it was segregation and racial discrimination they meant to defend.
In truth, Southerners were not on the verge of losing their slaves in 1861. Abolitionist sentiments were held by a majority of the voting population of the United States—that was true. And, yes, federal laws were clearly aimed at quarantining slavery in the South. But the way forward to actually emancipating slaves was not at all clear.
"The wicked flee when no man pursueth." The South feared it might lose its slaves sometime down the road, but Abolitionists really had no plausible way to enact their sentiments immediately or soon. Some historical economists have thought that slavery would have become economically nonviable in a generation or so, and slavery would have become vestigial institution, eventually sloughed off. Maybe. But in 1861 slavery was still immensely profitable. Moreover, without secession the political means of cutting off slavery entirely did not exist in 1861. Lincoln knew that. Republicans in Congress knew that. The Supreme Court knew that. Emancipation was not on the agenda. Yet the South felt it must flee the Union, fired on Fort Sumter, and sealed its fate.
That cowardice and wickedness is what is memorialized with every Confederate-flag license plate, every historical re-enactment, every bronze thing sacred to the memory of slavers in the South. Why should any of that be any more acceptable than if Germans had historical re-enactments of the Nuremberg rallies or sold swastika key chains at souvenir stands?