The Mexican War of 1846-1848, largely forgotten today, was the second costliest war in American history in terms of the p ercentage of soldiers who died. Of the 78, 718 American soldiers who served, 13, 283 died, constituting a casualty rate of 16.87 percent. By comparison, the casualty rate was 2.5 percent in World War I and World War II, 0.1 percent in Korea and Vietnam [TC: you’ll find better but still lower estimates here], and 21 percent for the Civil War.
That is from American Ulysses: A Life of Ulysses S. Grant, by Ronald C. White, a good book by the way. I had not known that a possible U.S. takeover of “Santo Domingo” (today’s D.R.) was such a big issue during Grant’s administration.