From your own link:
Tensions between Iraq and Iran were fueled by Iran's Islamic revolution and its appearance of being a
Pan-Islamic force, in contrast to Iraq's
Arab nationalism. Despite Iraq's goals of regaining the Shatt al-Arab,
[note 1] the Iraqi government seemed to initially welcome Iran's Revolution, which overthrew Iran's Shah, who was seen as a common enemy.
[53][59] It is difficult to pinpoint when tensions began to build, but there were frequent cross-border skirmishes,
largely at Iran's instigation.[14]
In 1975,
the Iraqis launched an offensive into Iran using tanks, though the Iranians defeated them.
[43] Several other attacks took place; however, Iran had the world's fifth most powerful military at the time and easily defeated the Iraqis with its
air force. As a result, Iraq decided against continuing the war, choosing instead to make concessions to
Tehran to end the Kurdish rebellion.
[53][57]
In the 1975 Algiers Agreement, Iraq made territorial concessions—including the Shatt al-Arab waterway—in exchange for normalised relations.[53] 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.
[53] Iraqis viewed the Algiers Agreement as humiliating.
[53][58]:260 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.
[58]:298 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."
[58]
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 this,
Saddam merely considered the Algiers Agreement to be a truce, rather than a definite settlement, and waited for an opportunity to contest it.