<P> 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 . </P>
how could competition for resources and markets lead to the wars of the twentieth century
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