Ba'athist Iraq believed that Iran would not respond effectively due to internal socio-political turmoil caused by the country's Islamic Revolution one year earlier.

[13][14] On 22 September, Iraqi aircraft pre-emptively bombarded ten Iranian airfields in an ultimately unsuccessful attempt to gain aerial superiority on the battlefield.

On the next day, Iraqi troops crossed the international border in strength and advanced into Iran in three simultaneous thrusts along a front of approximately 644 kilometres (400 mi).

Of Iraq's six divisions that were invading by land, four were sent to Iran's oil-rich Khuzestan in order to cut off Iranian access to the Shatt al-Arab and establish a territorial security zone.

[15] Iraqi president Saddam Hussein presented the invasion as a strategically defensive measure to blunt the edge of Iranian politician Ruhollah Khomeini, who had risen to power as Iran's "Supreme Leader" and was attempting to export the Islamic Revolution to the Arab world.

Saddam, as a secularist and an Arab nationalist, perceived Iran's Shia Islamism as an immediate and existential threat to his Ba'ath Party and thereby to Iraqi society as a whole.

[17][18][19][20] Saddam had also aspired to annex Khuzestan and saw the Islamic Revolution as an opportunity to do so, seeking to increase his country's prestige and power in the Arab world.

[20][21] To this end, his administration hoped that Iraq, as an Arab-majority country, could successfully exploit Arab separatism in Khuzestan to undermine Iran from within.

In 1979–1980, Iraq was the beneficiary of an oil boom that saw it take in US$33 billion, which allowed the government to invest heavily in both civilian and military projects.

[17] On several occasions, Saddam alluded to the Muslim conquest of Persia while promoting his country's position against Iran in the context of a looming war.

[44] Iraq also helped to instigate riots among Iranian Arabs in Khuzestan province, supporting them in their labor disputes, and turning uprisings into armed battles between Iran's Revolutionary Guards and militants, killing over 100 on both sides.

[51][52] However, rather than turning against the revolutionary government as experts had predicted, Iran's people (including Iranian Arabs) rallied in support of the country and put up a stiff resistance.

With the conclusion of the "liberating operations", on 17 September, in a statement addressed to Iraq's parliament, Saddam stated:The frequent and blatant Iranian violations of Iraqi sovereignty...have rendered the 1975 Algiers Agreement null and void...

[22]Despite Saddam's claim that Iraq did not want war with Iran, the next day his forces proceeded to attack Iranian border posts in preparation for the planned invasion.

[22] Iraq's 7th Mechanised and 4th Infantry Divisions attacked the Iranian border posts leading to the cities of Fakkeh and Bostan, opening the route for future armoured thrusts into Iran.

[17] Of Iraq's six divisions that were invading by ground, four were sent to Khuzestan, which was located near the border's southern end, to cut off the Shatt al-Arab from the rest of Iran and to establish a territorial security zone.

[17]: 23 On the central front, the Iraqis occupied Mehran, advanced towards the foothills of the Zagros Mountains, and were able to block the traditional Tehran–Baghdad invasion route by securing territory forward of Qasr-e Shirin, Iran.

[17]: 23 Two of the four Iraqi divisions which invaded Khuzestan, one mechanised and one armoured, operated near the southern end and began a siege of the strategically important port cities of Abadan and Khorramshahr.

[17] Reflecting the bloody nature of the struggle, Iranians came to call Khorramshahr "City of Blood" (خونین شهر, Khunin shahr).

[17] The battle began with Iraqi air raids against key points and mechanised divisions advancing on the city in a crescent-like formation.

They were slowed by Iranian air attacks and Revolutionary Guard troops with recoilless rifles, rocket-propelled grenades, and Molotov cocktails.

[60] On 30 September, Iran's air force launched Operation Scorch Sword, striking and badly damaging the Osirak nuclear reactor near Baghdad.

On 28 November, Iran launched Operation Morvarid (Pearl), a combined air and sea attack that destroyed 80% of Iraq's navy and all of its radar sites in the southern portion of the country.