The Shatt al-Arab 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 Treaty of Saadabad, 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 al-Arab's eastern side, except at Abadan and Khorramshahr, where the frontier ran along the thalweg (deepest part of the river valley). This gave Iraq control of most of the 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 Abd al-Karim Qasim, 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" and 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 Shatt al-Arab, 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.