Ooops, I goofed - more Americans were killed in WWII than the Civil War (but not by a huge amount). Given the populations and technology involved, though, the surprise is how close the casualities were...
Wrong.
The American Civil War was the deadliest war in American history, resulting in the deaths of 620,000 soldiers and an undetermined number of civilian casualties.
In compaison, 418,500 Americans (incl. 416,800 military personnel and 1,700 civilians) died as a result of World War II.
Regards,
Much of the disparity was due to the state of medical technology. Many of the wounded who died in the War Between the States would have been saved by WWII medical technology and converesely the numbers of WWII wounded would have been VASTLY higher with Civil War medicine. A more useful ratio is wounded vs dead, far higher in WWII. 96% of the wounded who made it alive to Battalion Aid and higher medical facilities in WWII survived.
They hadn’t even discovered (Not til the 1870’s) Joseph Lister’s germ theory which was instrumental in developing practical applications of of disease control (notably gangrene) with respect to surgical techniques.