The article analyses key factors of conflict in Iran-Iraq long-term relations through the prism of the historical deepening of contradictions between the two nations and states, which found their expression, first of all, in the framework of the territorial conflict regarding the ownership of the water area of the Shatt-el-Arab river, which is strategically important regarding to its geographical location. Some attention is paid to the role of the «Kurdish question» in the development of interstate relations; the religious dimension of the Iran-Iraq conflict is also analyzed in detail, the role of which increased rapidly after the events of the Islamic Revolution in Iran in 1979. The ideological and political strategies of both opponents in the struggle for regional leadership are considered in detail. The main groups of factors causing the Iran-Iraq conflict are separated and briefly characterized. Also, the article carefully describes the military-political grounds and the course of the key stages of the direct armed confrontation and its role in the contemporary Middle Eastern geopolitical picture, including objective reasons that forced various Arab and other actors of different levels to take their own position in the context of the struggle between the two regional "power centers". A change in the nature of the participation of the main international players involved in the conflict is observed, depending on the trends in the arena of military operations.