Make no mistake: the Confederacy revolted over the issue of slavery. The states of the Confederacy clearly stated so. The Union, however, was not legally on a crusade. They were justified in warring against the Confederacy because the Confederacy was an insurrection, and not only that, attacked a Union possession within its borders. But also have no doubt that beyond the legal justification which made the Union counter-attack necessary, the Union believed was willing to fight because of the moral horror at slavery. One only need listen to the Battle Hymn to understand the Union’s motivations.
The reasons why the Confederacy invoked the right to independence asserted by the Declaration of Independence is irrelevant. Only the reason why the Union decided to attack them matters, and the Union did not attack them because they had slavery. The Union attacked them because they were trying to become independent of the financial control of Washington and it's Crony Capitalists in New York.
"Slavery" is an ad hoc excuse to cover up the fact that the North attacked them because they didn't want to lose control of the South's economics.
They were justified in warring against the Confederacy because the Confederacy was an insurrection,
It was a democratically approved process of seeking independence from Washington DC. They took a vote. The people of those states voted to leave. It was all orderly and in accordance with long established principles of Democracy.
The powers that be in Washington didn't like the fact that they were trying to separate themselves from Washington's control, and they called it an "Insurrection."
One only need listen to the Battle Hymn to understand the Union’s motivations.
If the motivation of the Union was to destroy slavery which had been legal in the Union for "four score and seven years", then why didn't they attack Delaware, Maryland, Kentucky, or Missouri? Those were Union slave states, some of which kept slavery for six months longer than did the states of the Confederacy.
How can a Union which claims to be fighting against slavery (which they had no trouble keeping legal when they were in charge) be tolerant of slavery in their own states?
It's a lie. It's propaganda meant to fool gullible people.