In the first weeks of 1861, six Southern states began the process of establishing their own government, even as Northerners debated whether to let them leave the Union in peace or use force to stop them. On February 23, two weeks before Abraham Lincoln (1809–1865) was inaugurated as the sixteenth president of the United States, Texas became the seventh state to leave the Union. After taking office, President Lincoln reacted cautiously to these events. He felt very strongly that the states that had seceded (left the Union) had no right to do so, and he was determined to keep the Union together. But he also did not want to upset the large number of states in the mid-South—sometimes called the border states—that had yet to decide whether to join the Confederacy.

On April 12, 1861, however, South Carolina troops attacked Fort Sumter, a U.S...