"The traditional estimate has become iconic. It's been quoted for the last hundred years or more," Mr. Hacker says. "If you go with that total for a minute — 620,000 — the number of men dying in the Civil War is more than in all other American wars from the American Revolution through the Korean War combined. And consider that the American population in 1860 was about 31 million people, about one-tenth the size it is today. If the war were fought today, the number of deaths would total 6.2 million."