Russo-Turkish War of 1877-1878
ND – MPE: 0.27
The Wikipedia article called “Russo-Turkish War (1877-1878)” cites plausible estimates for the number of civilians killed in the war, which range from 0.22 to 0.32 million. Matthew White cites similar estimates. I choose 0.27 as the most plausible estimate. Britain intervened with warships.