That new actions or tactics carried out by armed forces in battle (battlefield practices) meant that the dead caused by the battle (the slain) were unlikely to be respected, easily identified, retrieved and buried. (proper treatment of the slain) The expectation previously being the dead would be respected, could be identified, retrieved and buried relatively easily. Hence ‘upend’. The change in battle practices that caused this could vary widely depending on the time period and country, but hopefully with this definition you can work it out by context.