A Union Divided.

Slavery, at the time of President Lincoln's term, was a heated topic. Secession began solely for the facts that the South believed that the election of Abraham Lincoln was putting their ability to own slaves at risk - therefore dramatically damaging their economy. Even in the North, ( to many's surprise) there were slavery supporters. For this reason, Lincoln essentially had to make the war about preserving the Union. If he made it about slavery, he would have lost supporters, which he greatly needed during the course of the war. Particularly if he was to gain a second term.