Abraham Lincoln
After Lincoln's election in 1860, seven States (South Carolina, Mississippi, Alabama, Florida, Georgia, Texas and Louisiana) formed the Confederate States of America. When the United States refused to surrender Fort Sumter in Charleston, South Carolina, the Confederates attacked the fort, beginning the American Civil War.  Later, four more states (Arkansas, Virginia, Tennessee, and North Carolina) joined the Confederacy for a total of eleven.  In his whole period as president, he had to rebuild the Union with military force and many bloody battles.  He also had to stop the "border states", like Kentucky, Missouri, and Maryland, from leaving the Union and joining the Confederacy.