620 million people died during the Civil War in the 1860s in the United States. Popular wisdom tells the myth that it was a war against slavery but the reality was that the southern states wanted to secede and form a country of their own where politicians from Washington wouldn't be telling them what to do. Slavery was at the center of the discussion but it was rather the desire to do as they please without Washington's intereference what was driving out the southern states. Since the northern states had practically no natural resources, such as cattle and other goods from the south to subsist, they were not very inclined to allow this separation. President Lincoln agonized over this but decided that was was the only way for the northern states to survive.
The rethoric in those days was familiar. The north was fighting to free the slaves in the south; the south was fighting to preserve their freedom.
By comparison, the number of americans who died in WW2 was 405,000. Considering the country's population in the respective periods of time, these numbers represent 0.29% in the 1940s and 1.78% in 1860s; 6 times more people died at the hands of each other than those who died in foreign soil.