On the eve of July 14, 1946, an unprecedented violent outburst erupted in the oil city of Abadan in southwestern Iran between local Arab tribesmen, members of the recently founded "Arab Tribal Union," and members of the formidable Tudeh-affiliated union, the Council of Federated Trade Unions of Iranian Workers and Toilers. Although seemingly the result of recent happenings, this incident was, in fact, the culmination of several processes which had originated at the turn of the twentieth century in Iran and particularly in Abadan. What were the causes of tension and rivalry between these two unions? Why were Arab tribesmen, the majority of whom lived in Abadan's hinterland, organized in a trade union? How did Arab sheikhs come to be union leaders? What were the relations between the city and its hinterland and what influences did the oil industry and the Iranian authorities have on the urban and tribal communities? These are the main questions this article tries to address.