There are observable realistic factors - economic, legal and political - that contribute to initiating and maintaining international conflicts, including starting wars. Behind such factors there are also various psychological elements. This paper recalls correspondence between Albert Einstein and Sigmund Freud and Freud’s pessimistic response to the question “Why War?” and explores this question from the point of view of large-group psychology. The term “large group” describes thousands or millions of people, most of whom never see or even know about each other as individuals, but who share many of the same sentiments and the same realistic or fantasied ancestral history. This paper defines a concept, “large-group identity”, and describes how protecting it plays a key role in maintaining and even initiating war-like situations or wars. This paper also considers the psychological make-up of a decision-maker with political power who encourages war.