As of 2015, the American war with the highest number of casualties is the U.S. Civil War. During the Civil War, 620,000 Americans died. World War II is the war with the next highest level of causalities, with 405,000 Americans losing their lives.
There was a total of 1.5 million casualties suffered during the Civil War. During the war, 620,000 people died, 476,000 were wounded, and 400,000 were captured or went missing. Nearly as many men died in captivity during the Civil War as died fighting in the Vietnam War. Roughly 2 percent of the U.S. population at the time perished during the Civil War.