The determining of the Western frontiers of Iran was a process that lasted almost 400 years. It indicates extensive maneuvers of the areas under conflict and interference of big powers, and Britain in particular. The intervention of Russia and the UK in determining the western boundaries of Iran since the mid-19th century resulted in the signing of several agreements regarding the demarcation of the Iran-Ottoman joint borders, which are presently the joint borders of Iran and Iraq, based on the same treaties, with little or no change. This article studies the process of determination of the Iran-Ottoman frontiers – and subsequently with Iraq as Iran's new western neighbor after World War I – since the conclusion of the Erzurum Treaty in 1847 till the signing of the Iran-Iraq Accord of 1975, with respect to the role of the two big powers and the UK in particular.

 Keywords: Khorramshahr, Arvand Roud, Sulaymaniah, Zahab, Thalweg