While some scholars see war as a universal and ancestral aspect of human nature, others argue it is a result of specific socio-cultural or ecological circumstances.
The deadliest war in history, in terms of the cumulative number of deaths since its start, is World War II, from 1939 to 1945, with 60–85 million deaths, followed by the Mongol conquests at up to 60 million. 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). In 2013 war resulted in 31,000 deaths, down from 72,000 deaths in 1990. In 2003, Richard Smalley identified war as the sixth (of ten) biggest problem facing humanity for the next fifty years. 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 the Byelorussian SSR 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). Another byproduct of some wars is the prevalence of propaganda by some or all parties in the conflict, and increased revenues by weapons manufacturers.
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”. In German, the equivalent is Krieg (from Proto-Germanic *krīganą 'to strive, be stubborn'); the Spanish, Portuguese, and Italian term for "war" is guerra, derived like the Old French term from the Germanic word.
War must entail some degree of confrontation using weapons and other military technology and equipment by armed forces employing military tactics and operational art within a broad military strategy subject to military logistics. Studies of war by military theorists throughout military history have sought to identify the philosophy of war, and to reduce it to a military science. Modern military science considers several factors before a national defence policy is created to allow a war to commence: the environment in the area(s) of combat operations, the posture national forces will adopt on the commencement of a war, and the type of warfare troops will be engaged in.
The earliest recorded evidence of war belongs to the Mesolithic cemetery Site 117, which has been determined to be approximately 14,000 years old. About forty-five percent of the skeletons there displayed signs of violent death. Since the rise of the state some 5,000 years ago, military activity has occurred over much of the globe. The advent of gunpowder and the acceleration of technological advances led to modern warfare. According to Conway W. 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)." An unfavorable review of this estimate 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. Primitive warfare is estimated to have accounted for 15.1 % of deaths and claimed 400 million victims. 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. For comparison, an estimated 1,680,000,000 people died from infectious diseases in the 20th century. Nuclear warfare breaking out in August 1988, when nuclear arsenals were at peak level, and the aftermath thereof, could have reduced human population from 5,150,000,000 by 1,850,000,000 to 3,300,000,000 within a period of about one year, according to a projection that did not consider "the most severe predictions concerning nuclear winter". 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.
In War Before Civilization, Lawrence H. Keeley, a professor at the University of Illinois, says approximately 90–95% of known societies throughout history engaged in at least occasional warfare, and many fought constantly.