Accounts of brutality fill the history of warfare. The behavior of any human being is, of course, a very complex phenomenon, whether in war or in peace. Historians in large part have described in detail the actions of military groups that have committed brutalities, but have not dealt with the factors that contributed to those actions. After examining the collective behavior of six military groups, representing different combat actions in different periods, some unexpected similarities became clear. While these groups were in very different situations and operated during different periods in history, there are similar factors that allowed the members of these groups to kill men, women and children in cold blood, and to commit acts of unspeakable brutality. After a close analysis of these military groups, five principle factors that had the greatest influence, either directly or indirectly, on these soldiers have been identified. Together, the factors supported each other and crystallized into a modus operandi that resulted in atrocities and bestial acts on civilians. This is the first book to identify the factors that lead to some of the most horrific cruelty in history, and to predict the actions of future groups given similar circumstances.