quality:correction, quality:corruption, information:classification, structural:summarization
History

Scientific evidence for warfare has come from settled, sedentary communities. Some studies argue humans have a predisposition for violence (chimpanzees, also great apes, have been known to kill members of competing groups for resources like food). A comparison across mammal species found that humans have a paleolithic adult homicide rate of about 2%. This would be lower than some other animals, but still high. However, this study took into account the infanticide rate by some other animals such as meerkats, but not of humans, where estimates of children killed by infanticide in the Mesolithic and Neolithic eras vary from 15 to 50 percent. Other evidence suggests that organized, large-scale, militaristic, or regular human-on-human violence was absent for the vast majority of the human timeline, and is first documented to have started only relatively recently in the Holocene, an epoch that began about 11,700 years ago, probably with the advent of higher population densities due to sedentism. Social anthropologist Douglas P. Fry writes that scholars are divided on the origins of possible increase of violence—in other words, war-like behavior:

There are basically two schools of thought on this issue. One holds that warfare... goes back at least to the time of the first thoroughly modern humans and even before then to the primate ancestors of the hominid lineage. The second positions on the origins of warfare sees war as much less common in the cultural and biological evolution of humans. Here, warfare is a latecomer on the cultural horizon, only arising in very specific material circumstances and being quite rare in human history until the development of agriculture in the past 10,000 years.

Jared Diamond in his books Guns, Germs and Steel and The Third Chimpanzee posits that the rise of large-scale warfare is the result of advances in technology and city-states. For instance, the rise of agriculture provided a significant increase in the number of individuals that a region could sustain over hunter-gatherer societies, allowing for development of specialized classes such as soldiers, or weapons manufacturers.

In academia, the idea of the peaceful pre-history and non-violent tribal societies gained popularity with the post-colonial perspective. The trend, starting in archaeology and spreading to anthropology reached its height in the late half of the 20th century. However, some newer research in archaeology and bioarchaeology may provide evidence that violence within and among groups is not a recent phenomenon. According to the book "The Bioarchaeology of Violence" violence is a behavior that is found throughout human history.

Lawrence H. Keeley at the University of Illinois writes in War Before Civilization that 87% of tribal societies were at war more than once per year, and that 65% of them were fighting continuously. He writes that the attrition rate of numerous close-quarter clashes, which characterize endemic warfare, produces casualty rates of up to 60%, compared to 1% of the combatants as is typical in modern warfare. "Primitive Warfare" of these small groups or tribes was driven by the basic need for sustenance and violent competition.

Fry explores Keeley's argument in depth and counters that such sources erroneously focus on the ethnography of hunters and gatherers in the present, whose culture and values have been infiltrated externally by modern civilization, rather than the actual archaeological record spanning some two million years of human existence. Fry determines that all present ethnographically studied tribal societies, "by the very fact of having been described and published by anthropologists, have been irrevocably impacted by history and modern colonial nation states" and that "many have been affected by state societies for at least 5000 years."

The relatively peaceful period since World War II is known as the Long Peace.