Do you not realize how ridiculous is this idea? They didn't need to "preserve" something that had already existed within the Union for "four score and seven years."
It was legal at that time and place in history. Lincoln offered them assurances that he would do nothing to endanger slavery, and he even went so far as to support the Corwin Amendment which would have made it permanent and impossible to destroy.
The Civil War was about the control of that money produced by those slaves. About 73% of all hard foreign currency earned by the US came from those Southern slave states, and this represented a huge amount of money for the United States at that time.
Lincoln made it clear they could have slavery, but what they could not have was financial independence from Washington D.C.
Even the London Spectator noted at the time of the Emancipation Proclamation that:
The principle is not that a human being cannot justly own another, but that he cannot own him unless he is loyal to the United States.
Despite the limited terms of the Emancipation Proclamation, it marked a decisive shift in the North's war aims, from simple preservation of the union to preservation plus the end of slavery.
If the South had instead remained in the union, they could have stymied abolition while undertaking gradual reforms that ameliorated the conditions of slavery. Doing so would have preserved the South's prosperity and political power.
Except that's not what the Corwin Amendment said. What it said was that any amendment to the Constitution ending slavery had to originate in a slave state and needed unanimous support of all states. Read it for yourself. It did not prevent any state from ending slavery on its own, as many had already done. (Or do you think that it was only a matter for the federal government to determine?) It also did not permit the expansion of slavery into the territories, which is why the south refused to consider it.
Lincoln's belief was that slavery should be held to where it already existed, and that in time those states would end it on their own without federal interference. The south realized that slavery was doomed unless it could expand, if for no other reason than to get rid of the booming slave population.
I have read through all the comments on this board (about 40), and am amazed at the utter historical ignorance being shared. When we talk about slavery “causing” the Civil War, that is like saying Comey’s letter caused Hillary’s defeat. A kernel of truth in a much more complicated set of occurrences. First, the NORTH sure as hell was NOT fighting over slavery. In fact, Congress passed a constitutional amendment in early 1861 that would have guaranteed that slavery would be legal in the South forever. The Republican Party was not an anti-slavery party; it was, in fact, an anti-EXPANSION of slavery party. Most people in the North could have cared less about slavery—like 98%. Republican politicians despised slavery b/c the 3/5th Compromise gave rural Southerners a numbers advantage in Congress over the more heavily (white) populated North. This pissed them off, bigly. Also, many in New England saw slavery as a black mark on America—not b/c slavery was evil, but b/c New Englanders saw mixing black blood with white blood the same way Hitler did. If the North was NOT fighting to free the slaves—and they most assuredly were not—then how could slavery be THE cause of the Civil War. I could go on and on....