Slavery began to flourish in the U.S. at the end of the 18th century with Eli Whitney's invention of the cotton gin. Whereas it would take one person a day to produce a pound of clean cotton, the cotton gin could produce 1000 lbs per day. Business boomed and the demand for slaves to work new cotton fields rose accordingly. Serious objections to slavery began as early as 1831 and the abolitionist movement in the North grew quickly. There was particular concern about whether new States entering the Union would be free or slave States. Some, like John Brown, added religious zealotry to the cause and his raid on the federal at Harper's Ferry in 1859 ended in failure. The Southern States were genuinely concerned that the Union would outlaw slavery altogether. There were 21 million people living in the North compared to only 9 million in the South, which included 4 million slaves. The turning point came in the election of 1860 which essentially became a referendum on slavery. The South reacted swiftly to the election of Abraham Lincoln, burning him in effigy. Between Lincoln's election and his inauguration, 7 southern States seceded from the Union. The expected war began with the attack on fort Sumter. At the time, the Union Army had only 17,000 men and only 2 generals had combat experience. Thus began a massive recruitment campaign on both sides. In July 1861, the first battle of the war took place at Manassas, Virginia just across the Potomac from Washington. It proved to be a disaster for the North.
—garykmcd