Simply put, the revolutionary invention by Eli Whitney Jr. (1765-1825) allowed cotton seeds to be removed from the cotton fibers quickly and efficiently, turning the once laborious process of picking cotton into a financial powerhouse. The invention did not help to alleviate the need for slaves to pick the cotton, however, and once it was recognized that the cotton gin could help to produce much greater amounts of cotton, plantation owners also saw the need for more slaves to keep up with the worldwide demand. Interestingly, slavery was on the decline before Whitney's invention, which was supposed to be a "labor-saving device." Although the gin (short for "engine") made it far easier to remove the cotton seeds than doing so by hand, the magnitude of greater cotton production was soon evident, requiring more slaves to handle the increased crops. Cotton exports multiplied by 200 times in less than two decades, earning it the name "King Cotton." 

Cotton exports from the U.S. boomed after the cotton gin's appearance--from less than 500,000 pounds in 1793 to 93 million pounds by 1810... It became the U.S.'s chief export, representing over half the value of U.S. exports from 1820 to 1860.

Production further grew from 750,000 cotton bales in 1830 to nearly 3 million bales by 1850. The growth of slavery followed suit, increasing from 700,000 slaves in the 1790s to 3.2 million in 1850. Southern cotton eventually totalled two-thirds of the world's market.