Iran–Iraq border

Two empires

Persia and the Ottoman Empire, Iraq’s predecessor here, first set a common boundary in the Treaty of Zohab in 1639. Towns went to one side or the other mostly by the loyalties of the people living there, so the line was vague.^[1] Wars moved it and later treaties restored it, in 1746 and again in 1823.^[1]

The second Treaty of Erzurum, signed in May 1847, drew a line in the Shatt al-Arab for the first time. Persia got Khorramshahr, the island of Abadan and the river’s eastern bank; the river itself was generally taken to stay Ottoman, but Persian ships could use it freely.^[1]

Surveyors spent decades on the rest. The Constantinople Protocol of 17 November 1913 described the whole boundary, and a commission from Persia, the Ottoman Empire, Russia and Britain marked it with pillars by October 1914.^[1] In the Shatt al-Arab it followed the low-water mark on the Persian bank, except near Khorramshahr.^[1]

Whose river?

Iraq, carved from three Ottoman provinces, came under a British mandate in 1920 and became independent in 1932.^[1] Quarrels over the Shatt al-Arab reached the League of Nations in 1934. A treaty of 1937 kept the 1913–14 line but moved it out to the deepest channel (the thalweg) off Abadan, so that Iran’s jetties there were no longer in Iraqi water.^[1]

Iran kept pressing for the thalweg along the whole river. In 1975, with Algeria’s President Boumediene mediating, the Shah and Saddam Hussein agreed at Algiers to re-mark the land boundary on the 1913–14 line and to put the river boundary in the thalweg.^[1] The treaty and its protocols were signed in Baghdad on 13 June 1975.^[1] Britannica describes the eastern half of the river as Saddam’s main concession, made in return for an end to Iranian meddling in Iraq.^[2]

“to delimit their river boundary according to the thalweg line”^[1]

War, and back to 1975

After Iran’s revolution of 1978–79, Saddam saw the 1975 deal as lopsided and Iran as weak.^[2] Iraqi forces invaded western Iran on 22 September 1980, though Iraq said the war had begun on 4 September with Iranian shelling of border posts.^[2] Iraq took Khorramshahr but not Abadan. By 1982 Iran had pushed the fighting back to a static front just inside and along Iraq’s border.^[2]

The war killed perhaps 500,000 people, and some estimates of all casualties reach two million.^[2] Iran accepted UN Security Council resolution 598, which called for both sides to withdraw to the internationally recognised boundaries, and a ceasefire began on 20 August 1988. Troops were withdrawn and relations restored only after a formal peace agreement on 16 August 1990.^[2]