“The United States Civil War began as an effort to save the Union, and ended in a fight to abolish slavery.....After initially striving to keep the slavery issue out of the war, the Northerners began enlisting blacks to assist them in the fight. Lincoln's Second Confiscation Act and the Militia Act, both of 1862, were significant in building the Northern military, because together they punished rebel slave holders and encouraged employment of blacks in the Union army. ...A final goal the Emancipation Proclamation accomplished was to discourage England and France from entering the war on the South's side. They were supplied with cotton and tobacco by the South, and wanted them to win. However, when the war became an argument over slavery, the European nations, who were opposed to human bondage, gave their support to the Union, ...“ 
In our first sequel we showed why, in addition to the altruism, the US Civil War may had been waged in part so that both, the Southern plantations owners and the former slaves, liberated to take on paid employment, pay import, income and wage-taxes to the US state starved for finances. I.e. although segregation of the black people continued for another 100 years, well into the late 1960's, their income taxes were, nevertheless, collected in a rather egalitarian manner – a good value deal for their freedom it was, some may argue.
But, was it really a clean cut and was the liberation of the slaves then automatic?
The most of the slaves found on the Confederate plantations captured by the Unionist armies were not immediately liberated. They were, actually, initially “confiscated”, together with the plantation's land by US state as punitive measure a slave and landowner's for support of Confederate separation. The slaves were put under control of the army and held in camps next to army camps. Attempts by some officers to liberate the slaves found resistance by none else than Lincoln himself  who retracted those officers decisions and made sure that slaves remained in the custody (and ownership) of the state. But that may not have been as bad as it may sound - many of those liberated were, on the other hand, midst the chaos of the war, left without work, food or medical help and died from starvation or from illness . Many of those more able males were mobilised into the army, sometimes by force, sometimes by their choice, either to liberate other fellow slaves, or, ironically, as the only chance of survival for them and their families on the army wages they received. But, was it all just about a combination of abolitionists’ altruism, mobilising wider public support for a Unionist causes aimed at maintaining the US and the import-tax-free access to the rich Southern markets for the recession-hit North? Or for improving the Federal State’s tax revenues from the South and its, now tax-liable, liberated workforce?
The mixed colour metaphors of the Civil War and their roots
Real life is not a myth and it does not follow straight linear argument path of the classic formal logic and there is usually more than one, single reason for significant events taking places in human history.
In fact, it does not take much to find out some darker, understated but still plausible, additional reasons for both the starting and waging the bloody Civil War not mentioned in the film and understated in the historiography either. They may have been overlooked because these additional roots of the conflict were hidden from the eyes under the grounds of the main battlefields.
The major battles of the Civil War were held on the grounds of the newly discovered oil fields and, already well known, coal rich Appalachian Mountains slavery states such as Virginia, Ohio borders and Kentucky:
"Petroleum became a major industry following the oil discovery at Oil Creek Pennsylvania in 1859" - . "During the American Civil War, the oil-producing region spread over much of western Pennsylvania, up into western New York State, and down the Ohio River valley into the states of Ohio, Kentucky, and the western part of Virginia (now West Virginia). ..... [and] other locations in the western Appalachian mountains, where oil was seeping to the surface...",.
Oil drilling started in 1859 and the first refinery in the US opened in Pennsylvania in 1861 – the very year the Civil War started. The oil drilling and production industry grew during the Civil War, in good part lead by Rockefeller family from its very start. Though oil did not have the economic importance then as it has had ever since the late 1800’s, with the development of cars and internal combustion engines, it was already a rather significant economic factor as a lamp and heating fuel and a lubricant for the both Northern industrial and Southern agricultural machines. It may then not be surprise that resource rich Kentucky, as the main war-time borderline, was perceived to be of key importance:
"President Abraham Lincoln ... declared "I hope to have God on my side, but I must have Kentucky." … In a September 1861 letter to Orville Browning, Lincoln wrote "I think to lose Kentucky is nearly the same as to lose the whole game. ... ” 
So, the Civil War was greatly about assuring freedom, however, not only for freedom of the black slave workforce but also for freeing unconditional access to the southern markets and the "black gold", the sources of the newly established oil industry and the coal mines rich seceded Appalachians states. And all of those sources were increasingly vital for the growing industries of both South and the North ... and, yes, to be rationally consistent, crucial for the new income tax revenues of the strong Federal state. 
The simplifying, contrasting colours of Baroque paintings have too often found their written and verbal metaphor in usually simplified and polarised public declarations of the motives of wars and of their actors, whilst abstracting the very many shades of grey that lie between them.
The 1976 Oscar winning film “Black and White in Colour” shows how African black population was instrumentalised and divided for sake of waging the WW1 battles over their native African soils. Spielberg’s film Lincoln, however, is covering-up hints of any conceivable instrumentalisation and the issue of black slavery. The film is instead painting-over and hiding any alternative images not in line with the familiar, by mean of media repetition manufactured consent, accepted, simplified myth. A myth in which altruism driven liberation of slaves was the sole and main purpose for the war rather than abolition becoming a likely instrument of the mercantilism-driven, market widening and wider tax imposing Civil War that killed 750,000.
It can be also said that such (mainly white) nation building, social engineering myth may have stirred an even higher level of hatred for the blacks among the Southerners who, ever since the Civil War, made the mistake of putting the blame on the blacks and their liberation as the main cause of the war that destroyed their families, livelihood and their traditional way of life, rather than seeing the cause in the mercantilist policy of the Northerners. However, despite those tragic stories or its patchy motivation background and implementation, we can certainly celebrate the incoming 150th anniversary of the 13th Amendment as a major milestone in recent human history.
Addendum: Is there a lesson for independence seeking Scotland?
What may be the relevance of such myth-making for today? Well, we can ask ourselves what could have become a morally acceptable pretext for any, though unlikely, move that the recession-hit and debt-ridden RoB (the Rest-of-Britain) could have undertaken in order to keep the Scottish taxes in its treasury and to retain its access to the vital and valuable North Sea oil resources in case the Scottish independence referendum had favoured the independence?
In absence of major human-rights or slavery issues, given their love for animals and the global concerns for sustainability of wild life, the protection of the EU's sustainable fishing quotas in the North Sea could have been a pretty good starting point for the British naval blockade of the North Sea ...and its oil rigs. The rest of Britain (RoB) could then have become the first country in history to intervene militarily for the sake of the protection of animals. In this case we are, of course, talking about protection of a particular, very valuable and nearly extinct type of the North Sea oily fish – the famous red herring - a very frequent delight served at Westminster and Washington dinner parties.
(Note: However, as far as the recent tragic events in oil rich regions of Middle East: Syria, Iraq and Libya evolve in front of our eyes and cameras, humour stops and tragedy prevails again. Any similarities of the patterns evolving there with the above story are purely intentional)
Disclaimer: This text is written as a construction of possible reality on basis of references taken from internet but whose validity author could not check or guarantee and the resulting text above has no claim to accurately present real events or characters. Author apologises for any offence the text may cause – it was not intentional but purely coincidental and the main aim of the text was to entertain its reader with a fictional vision of an alternative (but plausible) reality.
quoted from http://www2.coloradocollege.edu/Dept/HY/Hy243Ruiz/Research/civilwar.html
 Lincoln's father in law was himself a wealthy slave-owner and Lincoln did not want to antagonize and loose support from other slave-owners who remained in the Union so he initially prohibited liberation of slaves even in captured Confederate territories: (http://en.wikipedia.org/wiki/Abraham_Lincoln_and_slavery )
 Additional References: History of the Income Tax in the United States — Infoplease.com http://www.infoplease.com/ipa/A0005921.html#ixzz2KSNAFbJ0