Civil Wars are tricky things.
Normally in War, the end game should be to kill as many of your enemies as possible and take over their resources, but in a Civil War, you have to face the reality that after it's all over, all sides will be citizens of the same country again. There's just something distasteful about celebrating a victory over your defeated foe when they're also your fellow citizens.
In Imperial Rome, they decreed that no victorious general was to receive a triumph (like a really, really cool parade with dancers, exotic animals and defeated enemies in chains) when the victory was over a rival for the throne. Then again, this was a little different than the American Civil War. in Rome- they were all striving for the same thing- control of an empire. It wasn't as though one rival wanted to break away from the empire while another wanted him to stay put.
This attitude of the southern states that they didn't consider themselves even to be American anymore opened the door for a level of brutality that probably surprised both sides.
Furthermore, most of the upper military class were either southerners themselves or were from border states. Also, even the northern military leaders often had sympathy to the southern cause themselves because they would generally receive their military training in the south and would feel a certain kinship with the south's aristocratic social order since they were often aristocrats in their own right.
Hardcore hatred from both sides, a halfway committed military and the reality that after it was all over, the north would basically have to rebuild what it destroyed in the south and remake the entire power and social structure of half the country. This is what Lincoln walked into. Also- Lincoln had no formal military training and basically learned military tactics on the job, reading the "Art of War" by candlelight after his hectic days in the office.
It's really amazing that he won the war.
I shudder to think what would have happened then and for the following 150 years of American history and feel that it shows Lincoln is worth a hell of a lot more than a penny.