While a case can be made that slavery was a huge issue in the war, the fact is that slavery became an issue because of it's economic importance to Southern industry, which I'm sure you know was agrarian while the North was industrial. Without slaves, the South would be industrially insolvent. Many southerners had intentions of turning their plantations into the more profitable industrial ventures of the north, but at that present time they were stuck in a way of life dependant upon slave labor. Without it, it would mean the total destruction of their capital base.
I would submit that although I agree slavery was a relevant issue of the time in regards to humanitarian principles, the issues of the Civil War had nothing to do with humanitarian principles and everything to do with the preservation of the union and the economics there within.
The South, being predominantly agricultural, had to import practically all of its manufactured goods from the Northern states or from Europe, both of which reciprocated by providing a market for the south's cotton. Goods from Europe were considerably cheaper than those from the North, even after the cost of shipping and such. Therefore, southern states often purchased goods from Europe, putting considerable competitive pressure on American manufacturers to lower their prices and operate more efficiently.
The Republicans were not satisfied with that arrangement and used the power of the federal government to tip the scales of competition in their favor. Claiming it was in the national "interest", they levied stiff import duties on almost every item coming from Europe that was manufactured in the North. Not surprisingly, there was no duty applied to cotton, which presumedly, was not a commodity in the national interest. European countries countered by stopping the purchase of U.S cotton, which badly hurt the southern economy. The result was manufacturers in the North were able to charge higher prices without fear of competition, and the South was forced to pay more for the majority of its goods. It was a case of legalized plunder where the North was enriching itself at the expense of the South.
In the beginning of the war, the North was not doing well, and in the early years, the outcome was far from certain. Numerous defeats and sagging enthusiasm decreased popularity for a war that was initially perceived as a war of business interests, and not of humanitarian principles. That presented two problems for the North.The first was how to get people to fight, and the second was how to get people to pay. Both problems were solved by the simple expediency of violating the Constitution, hence the Emancipation Proclomation. To get people to fight, it was decided to convert the war into an anti-slavery crusade. Preservation of the Union was not enough to fire men's enthusiasm for the war. To make the cause of freedom synonomous with the cause of the North, there was no alternative but to officialy decalre against slavery. After having emphasized over and over again that slavery was not the reason for war, Lincoln later explained why he changed his course and issued the proclomation:
"Things had gone from bad to worse until I felt we had reached the end of our rope on the plan we were pursuing; that we had about played our last card, and must change our tactics or lose the game. I now determined upon the adoption of the emancipation proclomation."
Although there was initial success with this strategy, once the bloodletting ensued, enthusiasm waned, and conscription was invoked on both sides. It is interesting to note that Lincoln not only had the South to deal with, but after conscription was forced, he had mass insurrections in the North as well which led to his yet again unconstitutional move of suspending habeas corpus to quell the rebellions.
