In 1860, the total population of the United States was 31.4 million;
If there were another Civil War today, and those same percentages were still true, then:
Another way to illustrate this point is that the death toll at Gettysburg, measured as a percentage of the nation's population, was 21 times that of the terrorist attacks of September 11th, 2001. In fact, measured as a comparative percentage of the American population, there were 42 Civil War battles in which the death toll exceeded that of September 11th, or almost one a month, for four long years. One cannot even begin to comprehend how the nation could cope with such a horrific and prolonged struggle today.
See, for example, William F. Fox, Regimental Losses in the American Civil War (Albany, NY: Albany Publishing Company, 1889). Confederate casualty data is incomplete for 1864 and 1865. The comparisons above do not include any Confederate losses from the sieges of Richmond and Petersburg, which would undoubtedly increase the number of comparable battles.