Final answer:
The Southern States with the highest percentage of slaves at the start of the Civil War were Alabama, Georgia, and Mississippi due to the importance of slavery in the economy of the Deep South.
Step-by-step explanation:
The Southern States that had the highest percentage of slaves at the start of the U.S. Civil War were in the Deep South, notable for having large numbers of both slaveholders and enslaved people. These states were Alabama, Georgia, and Mississippi. A greater concentration of slaves was found in these regions due to the heavy reliance on labor-intensive cash crops like cotton, which was given new life through innovations such as Eli Whitney's cotton gin. The heavy presence of slavery in these states also contributed to the political tensions leading up to the Civil War, as the Southern economy and way of life were fundamentally intertwined with the institution of slavery.