- World War II
- 56400000 people
By far the most costly war in terms of human life was World War II (1939–45), in which the total number of fatalities, including battle deaths and civilians of all countries, is estimated to have been 56.4 million, assuming 26.6 million Soviet fatalities and 7.8 million Chinese civilians were killed. The country that suffered most in proportion to its population was Poland, with 6,028,000 or 17.2 per cent of its population of 35,100,000 killed.
In the Paraguayan war of 1864–70 against Brazil, Argentina and Uruguay, Paraguay's population was reduced from 407,000 to 221,000 survivors, of whom fewer than 30,000 were adult males.