War and similar conflicts have long been an activity of humans, and is referred to in the earliest historical records. Wars involving hunter-gatherer and early agricultural cultures included clashes between clans and village groups. This sometimes led to the deaths of some participants, as has been observed up to the present century in such places as New Guinea and Borneo. In marked contrast, modern warfare involving advanced technological societies can wreak a truly awesome destruction—about 84 million people have been killed during the wars of the twentieth century.

The conflict that has been the most destructive to human life was World War II (1939-1945), during which about 38 million people were killed. World War I (1914-1918) resulted in about 20 million deaths, the Korean War (1950-1953) about 3 million, and the Vietnam War (1961-1975) about 2.4 million.

In addition to having awful consequences for people and...