Complex sentence: During the war, tens of thousands of slaves would escape, having a substantial economic effect on the American South: an estimated 25,000 in South Carolina; 30,000 in Virginia, and one-quarter of the slaves in Georgia.
Split into simpler sentences: During the war, tens of thousands of slaves escaped, having a substantial economic effect on the American South. An estimated 25,000 slaves escaped in South Carolina; 30,000 in Virginia, and one-quarter of the total slave population in Georgia.