Iran had repudiated the demarcation line established in the Persian Gulf in the Anglo-Ottoman Convention of 1913, arguing that the Iran–Iraq border in the Shatt al-Arab should be demarcated according to the thalweg principle. The Kingdom of Iraq, encouraged by Britain, took Iran to the League of Nations in 1934, but the dispute was not resolved. In 1937, Iran and Iraq signed their first official boundary treaty. The treaty established the waterway border on the eastern bank of the river except for a four-mile anchorage zone near Abadan, which was allotted to Iran and where the border ran along the thalweg. In 1958, the Royal monarchy of Iraq was overthrown by a coup d'etat led by Iraqi leftists and nationalists coup d'état. 10 years later, in 1968, Arab Socialist Ba'ath Party took power after leading the coup d'état, which established the re-establihing the Ba'ath regime in Iraq. Shortly afterwards, Iran sent a delegation of diplomats to Iraq in 1969, and when the Iraqi government refused to proceed with negotiations over a new treaty, Iran withdrew the treaty of 1937. 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.