The Arab world today is encountering a destructive resurgence of sectarianism, which, up to a few years ago, had been confined to books and rhetorical debates. In the first half of the 20th century, Iraqi sociologist Ali Al-Wardi pioneered the critique of sectarianism in the Arab world. Unlike others, he approached the issue from a specific and unique perspective. His observations of Iraqi history were made from a sociological standpoint that aimed at revealing the impact of sectarianism on Iraqi politics. Al-Wardi's writings were carried out to two phases: the first extends from the early 1950s to the early 1960s; and the second covers the period between the publication of his two books, Study on the Nature of Iraqi Society (the year of publication is unknown) and Social Briefs from the Modern History of Iraq (1971). The first phase focused on Islam's heritage and a number of social phenomena; the second focused exclusively on the study of Iraqi society. A number of factors influenced Al-Wardi's personality and thinking, a fact especially evident in the kind of methodology he used, which was new when addressing the sectarian issue. Based on Al-Wardi's research, this paper traces the historical factors and process that affected the historical development of the divisions separating two main sects of Islam – Shi'a and Sunni – resulting in a duel between them, which led to the configuration of Iraqi society along sectarian lines. It elaborates on the methodology used by Al-Wardi in his studies of Iraqi society, as well as his attitude with regard to sectarianism in modern Iraq. It also explores the intellectual and political influences that helped shape his thinking in this domain and its legacy on sociological thought in the Arab world.