What is the theme in With Malice Toward None: The Life of Abraham Lincoln by Stephen B. Oates?

Unity is another theme in the book. The South's decision to secede from the Union threatened everything America had been built upon: the Constitution did not bend to suit the will of the few, and secession was unconstitutional. The nation had to stand together as one, or it would fall, divided. Lincoln focused his efforts in the Civil War on preserving the Union. He and his cabinet thought the South had no right to secede, and that the American experiment of self-government could not go down in flames. The people had to learn to live together and accept changes the nation was undergoing. Keeping the Union whole was a cause to die for, though some also fought for Emancipation.