Eli Whitney's invention of the cotton gin helped lead to the Civil War by making it possible to produce more cotton, thus increasing the profitability of huge cotton plantations in the South. These large plantations needed large numbers of workers in order to operate. Slavery helped meet those needs.
The use of the cotton gin allowed an almost four-fold increase in the production of cotton in the United States. The climate of the Southern states was more conducive to growing large crops of cotton. Many farmers increased the size of their fields and established large cotton plantations. Because slavery was legal in the South, they manned the plantations with slaves. Slavery increased 500 percent during this period, and plantation owners felt they could not function without slaves. By 1860, the United States produced about two-thirds of the cotton used in the world. The economy of the South depended almost exclusively on the cotton crop. In the meantime, Congress grappled with laws regarding which states just entering the Union were slave states and which were free. With the election of Abraham Lincoln to the presidency, Southerners feared their world crumbling. They knew Lincoln's anti-slavery stance, and they knew their present way of living was doomed if slavery was outlawed. The South decided to secede, and the Civil War became inevitable.