Shatt al-Arab
In April 1969, Iran abrogated the 1937 treaty over the Shatt al-Arab and Iranian ships stopped paying tolls to Iraq when they used the Shatt al-Arab. The Shah argued that the 1937 treaty was unfair to Iran because almost all river borders around the world ran along the "thalweg", and because most of the ships that used the Shatt al-Arab were Iranian. Iraq threatened war over the Iranian move, but on 24 April 1969, an Iranian tanker escorted by Iranian warships (Joint Operation Arvand) sailed down the Shatt al-Arab, and 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 Accords of 1975.