By: Mir M.Hosseini
During the Mandate of Iraq (1920-32), the British advisors in Iraq were able to keep the Arvand Rood waterway bi-national under the principle that worked in Europe for Danube River: the dividing line was a line drawn between the deepest points along the stream bed. In 1937, Iran and Iraq signed a treaty that settled the dispute over control of the Arvand Rood. The 1937 treaty recognized the Iranian-Iraqi border as along the low-water mark on the eastern side of the Arvand Rood except at Abadan and Khorramshahr where the frontier ran along the Thalweg (the deep water line) which gave Iraq control of almost the entire waterway; provided that all ships using the Arvand Rood fly the Iraqi flag and have an Iraqi pilot, and required Iran to pay tolls to Iraq whenever its ships used the Arvand Rood.
By the late 1960s, the build-up of Iranian power under Mohammad Reza Shah Pahlavi led Iran to take a more assertive stance in the Near East. In April 1969, Iran abrogated the 1937 treaty over the Arvand Rood, and as such, Iran ceased paying tolls to Iraq when its ships used the Arvand Rood. The Shah justified his move by arguing that almost all river borders all over the world ran along the thalweg, and by claiming that because most of the ships that used the Arvand Rood were Iranian, the 1937 treaty was unfair to Iran.
Iraq threatened war over the Iranian move, but when on 24 April 1969 an Iranian tanker escorted by Iranian warships sailed down the Arvand River, Iraq being the militarily weaker state did nothing. The Iranian abrogation of the 1937 treaty marked the beginning of a period of acute Iraqi-Iranian tension that was to last until the Algiers Treaty of 1975.