Final answer:
Seven slave states seceded before Fort Sumter; four more seceded after, while four remained in the Union.
Step-by-step explanation:
Before the siege of Fort Sumter, seven slave states had seceded: South Carolina, Mississippi, Florida, Alabama, Georgia, Louisiana, and Texas. After the siege, four more states seceded: Virginia, Arkansas, Tennessee, and North Carolina.
Following President Abraham Lincoln's call for volunteers to suppress the rebellion after Fort Sumter, additional slave states made the decision to secede. Virginia, Arkansas, Tennessee, and North Carolina joined the Confederacy, whereas Delaware, Maryland, Missouri, and Kentucky remained in the Union. This escalation led to a full-scale civil war, largely predicated on the issues of states' rights and slavery.