The Civil War continues to be the most deadly American war, but the approximately 600,000 deaths could actually be an undercount. The New York Times' Disunion blog:
New estimates, based on Census data, indicate that the death toll was approximately 750,000, and may have been as high as 850,000.
Nearly a million Americans died because of southern secession. If there's a modern lesson in this, it's that the idea of seceding ought to be taken very, very seriously and anyone who dabbles in the notion is a colossal idiot with an unhealthy degree of bloodlust.