Initially the war was fought to preserve the Union, then Lincoln included freeing the slaves. Today, I reckon this would be called mission creep.
Why do you think the war was fought?
It was initially about the right of secession, and the right of sovereign States to determine their own destiny (States rights). But then seeing as how Lincln didn't believe in State Sovereignty (in contravention to the Founders beliefs) and slavery was a political hot potato, Lincoln being a consummate liar (i.e. politician) decided (to gain popularity and support for his aggressive invasion of the Southern States) that he needed to emancipate the slaves. This of course set well with the die-hard Yankee abolitionists. And so it goes.
Seems to suggest the civil war has descended into a quagmire and GWB has no exit strategy for it. LOL ;-)