Answers:
1. Abolitionists celebrated when the war began because it meant that the Confederacy had chosen to fight for their right to keep slavery, and that the Union had the chance to end it.
2. Lincoln thought the border states would secede because they were some of the only states that still allowed slavery and had threatened to join the Confederacy if it declared independence.
3. Lincoln was trying to emphasize that the war was being fought not just to preserve the Union, but also to put an end to slavery. He wanted to make sure that everyone understood the importance of this issue, and that it was not just a political dispute.