 Complete the following phrase:  Iran had repudiated the demarcation line established in the Persian Gulf in the Anglo-Ottoman Convention of 1913, arguing that the IranIraq 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 detat led by Iraqi leftists and nationalists 