As concerns a belligerent's losses in proportion to its prewar population, the most destructive war in modern history may have been the Paraguayan War (see Paraguayan War casualties).

The English word war derives from the late Old English (circa.1050) words wyrre and werre, from Old French werre (also guerre as in modern French), in turn from the Frankish *werra, ultimately deriving from the Proto-Germanic *werzō 'mixture, confusion'.

The word is related to the Old Saxon werran, Old High German werran, and the German verwirren, meaning “to confuse”, “to perplex”, and “to bring into confusion”.

Total war is warfare that is not restricted to purely legitimate military targets, and can result in massive civilian or other non-combatant casualties.

While some scholars see war as a universal and ancestral aspect of human nature, which was greater than 41 million.

This would have been a proportional reduction of the world’s population exceeding the reduction caused in the 14th Century by the Black Death, and comparable in proportional terms with the plague’s impact on Europe's population in 1346–53. Keeley, a professor at the University of Illinois, says approximately 90–95% of known societies throughout history engaged in at least occasional warfare, Keeley describes several styles of primitive combat such as small raids, large raids, and massacres.

All of these forms of warfare were used by primitive societies, a finding supported by other researchers.War usually results in significant deterioration of infrastructure and the ecosystem, a decrease in social spending, famine, large-scale emigration from the war zone, and often the mistreatment of prisoners of war or civilians.For instance, of the nine million people who were on the territory of Soviet Belarus in 1941, some 1.6 million were killed by the Germans in actions away from battlefields, including about 700,000 prisoners of war, 500,000 Jews, and 320,000 people counted as partisans (the vast majority of whom were unarmed civilians).The advent of gunpowder and the acceleration of technological advances led to modern warfare. Henderson, "One source claims that 14,500 wars have taken place between 3500 BC and the late 20th century, costing 3.5 billion lives, leaving only 300 years of peace (Beer 1981: 20)." mentions the following regarding one of the proponents of this estimate: «In addition, perhaps feeling that the war casualties figure was improbably high, he changed "approximately 3,640,000,000 human beings have been killed by war or the diseases produced by war" to "approximately 1,240,000,000 human beings...&c.".» The lower figure is more plausible, but could also be on the high side, considering that the 100 deadliest acts of mass violence between 480 BCE and 2002 CE (wars and other man-made disasters with at least 300,000 and up to 66 million victims) claimed about 455 million human lives in total.Added to the aforementioned (and perhaps too high) figure of 1,240 million between 3500 BC and the late 20th century, this would mean a total of 1,640,000,000 people killed by war (including deaths from famine and disease caused by war) throughout the history and pre-history of mankind.War is a state of armed conflict between societies.