Relation between Iran and Iraq

Since the Ottoman–Persian Wars of the 16th and 17th centuries, Iran (known as Persia prior to 1935) and Iraq fought over full control of the Arvand Roud/Shatt al-Arab waterway. Historically, the waterway (called Arvand Roud in Iran and Shatt al-Arab in Iraq) and Khuzestan Province were all that remained of Iran's prior holdings in Mesopotamia, which had been lost to Turkey centuries earlier.Saadabad Pact, and relations between the two states remained good for decades afterwards.

 The Arvand Round was considered an important channel for both states' oil exports, and in 1937, Iran and the newly independent Iraq signed a treaty to settle the dispute. In the same year, Iran and Iraq both joined the Saadabad Pact, (The Treaty of Saadabad (or the Saadabad Pact) was a non-aggression pact signed by Turkey, Iran, Iraq and Afghanistan on July 8, 1937. This treaty lasted for five years. The treaty was signed in Tehran's Saadabad Palace and was part of an initiative for greater Middle Eastern-Oriental relations spearheaded by King Mohammed Zahir Shah of Afghanistan. Ratifications were exchanged in Tehran on June 25, 1938 and it became effective on the same day. It was registered in League of Nations Treaty Series on July 19, 1938) and relations between the two states remained good for decades afterwards.

The 1937 treaty recognised the Iran-Iraq border to be along the low-water mark on the Shatt's eastern side, except at Abadan and Khorramshahr, where the frontier ran along the deep water line (thalweg). This gave Iraq control of most of waterway and required Iran to pay tolls whenever its ships used it.
In 1955, both nations joined the Baghdad Pact. However, the overthrow of the Hashemites in Iraq in 1958 brought a nationalist government to power which promptly abandoned the pact. On 18 December 1959, Iraq's new leader, General Abdul Karim Qassim, declared: "We do not wish to refer to the history of Arab tribes residing in al-Ahwaz and Mohammareh [Khorramshahr]. The Ottomans handed over Mohammareh, which was part of Iraqi territory, to Iran." The Iraqi government's dissatisfaction with Iran's possession of the oil-rich Khuzestan province (which the Iraqis called Arabistan) that had a large Arabic-speaking population was not limited to rhetorical statements. Iraq began supporting secessionist movements in Khuzestan, and raised the issue of its territorial claims at an Arab League meeting, though unsuccessfully.
Iraq showed reluctance in fulfilling existing agreements with Iran—especially after Egyptian President Gamal Abdel Nasser's death in 1970 and the Iraqi Ba’ath Party's rise which took power in a 1968 coup, leading Iraq to take on the self-appointed role of "leader of the Arab world". At the same time, by the late 1960s, the build-up of Iranian power under Shah Mohammad Reza Pahlavi, who had gone on a military spending spree, led Iran to take a more assertive stance in the region.
In April 1969, Iran abrogated the 1937 treaty over the Arvand Roud, and as such, ceased paying tolls to Iraq when its ships used the waterway. The Shah justified his move by arguing that almost all river borders around the world ran along the thalweg, and by claiming that because most of the ships that used the waterway were Iranian, the 1937 treaty was unfair to Iran. Iraq threatened war over the Iranian move, but when, on 24 April 1969, an Iranian tanker escorted by Iranian warships sailed down the river, Iraq—being the militarily weaker state—did nothing.
Iran's abrogation of the treaty marked the beginning of a period of acute Iraqi-Iranian tension that was to last until the Algiers Accords of 1975. In 1969, Saddam Hussein, Iraq's deputy prime minister, stated: "Iraq's dispute with Iran is in connection with Khuzestan, which is part of Iraq's soil and was annexed to Iran during foreign rule." Soon, Iraqi radio stations began exclusively broadcasting into "Arabistan", encouraging Arabs living in Iran and even Balūchīs to revolt against the Shah's government. Basra TV stations began showing Iran's Khuzestan province as part of Iraq's new province Nasiriyyah, renaming all its cities with Arabic names.
In 1971, Iraq (now under Saddam's effective rule) broke diplomatic relations with Iran after claiming sovereignty rights over the islands of Abu Musa, Greater Tunb, and Lesser Tunb in the Persian Gulf following the withdrawal of the British. As retaliation for Iraq's claims to Khuzestan, Iran became the main patron of Iraq's Kurdish rebels in the early 1970s, giving the Iraqi Kurds bases in Iran and arming the Kurdish groups. In addition to Iraq fomenting separatism in Iran's Khuzestan and Balochistan provinces, both states encouraged separatist activities by Kurdish nationalists in the other state. From March 1974 to March 1975, Iran and Iraq fought border wars over Iran's support of Iraqi Kurds. In 1975, the Iraqis launched an offensive into Iran using tanks, though the Iranians defeated them. Several other attacks took place; however, Iran had the world's fifth most powerful military at the time and easily defeated the Iraqis with their air force. As a result, Iraq decided against continuing the war, choosing instead to make concessions to Tehran to end the Kurdish rebellion.

Iranian Cobra helicopters of the Imperial Iranian Airforce, circa 1970's

In the 1975 Algiers Agreement, Iraq made territorial concessions—including the Shatt al-Arab waterway—in exchange for normalised relations. In return for Iraq recognising that the frontier on the waterway ran along the entire thalweg, Iran ended its support of Iraq's Kurdish guerrillas. Iraqis viewed the Algiers Agreement as humiliating. However, the agreement meant the end of Iranian and American support for the Peshmerga, who were defeated by Iraq's government in a short campaign that claimed 20,000 lives. The British journalist Patrick Brogan wrote that "...the Iraqis celebrated their victory in the usual manner, by executing as many of the rebels as they could lay their hands on."

The relationship between the governments of Iran and Iraq briefly improved in 1978, when Iranian agents in Iraq discovered plans for a pro-Soviet coup d'état against Iraq's government. When informed of this plot, Saddam ordered the execution of dozens of his army's officers and in a sign of reconciliation, expelled Ruhollah Khomeini, an exiled leader of clerical opposition to the Shah, from Iraq. Despite that, Saddam merely considered the Algiers Agreement to be a truce, rather than a definite settlement and waited for the opportunity to contest it.