The most important dispute was over the Shatt al-Arab waterway. Iran repudiated the demarcation line established in the Anglo-Ottoman Convention of Constantinople of November 1913. Iran asked the border to run along the thalweg, the deepest point of the navigable channel. Iraq, encouraged by Britain, took Iran to the League of Nations in 1934, but their disagreement was not resolved. Finally in 1937 Iran and Iraq signed their first boundary treaty. The treaty established the waterway border on the eastern bank of the river except for a anchorage zone near Abadan, which was allotted to Iran and where the border ran along the thalweg. Iran sent a delegation to Iraq soon after the Ba'ath coup in 1969 and, when Iraq refused to proceed with negotiations over a new treaty, the treaty of 1937 was withdrawn by Iran. 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.