American Wars of Independence
The two major wars in North America — the "American Revolution" and the "American Civil War" — are both misnamed. A civil war is a struggle between two or more factions for control of one country: all participants regard themselves as citizens of the same country, and the winner will govern the whole country. A revolution is a civil war in which one faction aims to overturn not merely the government but the existing social order.
The French Revolution was a true revolution; the English Civil War and the Spanish Civil War were true civil wars. The wars that formed the United States of America and destroyed the Confederate States of America were neither civil wars nor revolutions: they were wars of independence. In the 1770s the colonists in America were not trying to overturn the British government or take control of Britain; they were merely trying to gain their independence. In the 1860s the Confederate States were not trying to overturn the US government or take control of the United States; they were merely trying to gain their independence.
In both cases, if the existing government had immediately recognized the independence of the new country, there would have been no war.
The original War for American Independence is relatively uncontroversial today. The British, who have given away so many parts of their empire, seem to accept placidly the early loss of potentially the most important part. (Though it can be interesting to speculate on the course of history had the British government of the time handled matters more skilfully.)
The War for Southern Independence is more recent and more difficult to evaluate morally and politically, as well as being an interesting conflict from a military point of view. For all these reasons, I've taken an interest in it, read books on it, and discussed it online.
Although many of the people involved in the war were doubtless fine individuals who took part for what seemed to them the best of motives, the mass killing of other people is never an admirable business, and in evaluating the two sides I find myself listing the defects of each cause. Of course it would have been much better for everyone if the politicians on each side had negotiated and reached a diplomatic settlement without war.
Defects of the US cause
The prime US defect was of course that the USA was fighting to conquer the CSA by military force and subject its people to US government against their will. This deserves the same condemnation as any other aggressive war of conquest.
Northerners commonly described the southerners as "rebels" and "traitors", and alleged that they were in breach of the US Constitution. In my opinion this is all nonsense. I've read the US Constitution, and it doesn't even mention secession. It's silly to claim that an action is illegal when there's no law against it. The Confederates seceded in a reasonable and democratic manner and elected their own democratic government. True, black people had no vote in the Confederacy; but at that time they had no vote in most of the northern states either.
Defects of the CS cause
The prime CS defect was of course that the CSA had sought independence mainly because it felt that its large-scale use of slave labour was under threat from the newly-elected US government. Confederates were fighting for their own liberty while denying it to their slaves.
There was a further defect worth mentioning. For decades white southerners had been accustomed to see themselves as a ruling class: ruling absolutely over their slaves, and dominating the US government for much of its existence. They had become arrogant and accustomed to getting their own way, and when war loomed they had no doubt they could win it, even though they were outnumbered by more than two to one and had far less industry capable of supporting a war. Their over-confidence led them to start the war by firing the first shots, and also led them to stop cotton exports to try to coerce Britain into supporting them. Both actions were belligerent and unwise — even though the first shots were fired under some provocation (the US occupation of a fort guarding Charleston harbour) and the stoppage of cotton exports was neither illegal nor immoral.
In principle, the arrogant and belligerent attitude of some Confederates doesn't justify the war. In practice, it may go some way towards explaining the northern willingness to fight.
Summing up
Both sides could claim that they were, in their different ways, fighting for liberty, which makes it difficult to say that one side was Right and the other Wrong.
If the USA had fought the war explicitly to free the slaves, I'd be willing to say that the USA was Right. However, in fact most northerners weren't interested in the plight of the slaves and wouldn't have fought for them. They fought to keep the southern states in the Union by force — in other words, to govern without the consent of the governed. This was the Wrong reason and leaves me unable to back either side whole-heartedly. Regrettably, both sides were enemies of liberty — in their different ways.
Abraham Lincoln, 1862: “My paramount object in this struggle is to save the Union... 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.”
Note on land ownership
I found in Grant's memoirs the argument that the entire territory of some American states was originally bought by the US government from foreign governments, and so for these states to secede would be “ingratitude and injustice”.
Well, even by 19th-century standards this wasn't really an enormous problem. The Louisiana Purchase of 1803 cost the US government $15 million, and much less than half of the purchased territory later turned Confederate. For comparison, the war is estimated to have cost the Confederate states billions of (US) dollars.
If the two sides had been willing to negotiate seriously, the Confederacy could easily have afforded to repay to the USA a reasonable proportion of the $15 million. Even with interest added, it would have been much cheaper than war.