The Iran-Iraq War is one of the major defining Middle Eastern events of the last half-century. Taken in conjunction with the Islamic Revolution of 1979, which is unavoidable given that one event immediately precipitated the other, it is the basis upon which a considerable part of modern Iranian politics is set. Oh, and its outcome also led directly to the Gulf War and America’s 20+ year obsession with taking out Saddam Hussein. And on this date in 1980, the war began with a series of Iraqi airstrikes against Iranian airfields, in preparation for a large ground invasion that was coming the next day.
At its core, the war was about two things: waterway rights in the Shatt al-Arab basin and Saddam’s fears about the Islamic Revolution. The Shatt al-Arab is a river that runs from the confluence of the Tigris and Euphrates Rivers out to the Persian Gulf, skirting the Iran-Iraq border for the last half (give or take) of its journey. Control over the river and its right of passage was a source of regular conflict between the Ottoman Empire (which controlled Iraq from the mid 16th century on, save for a couple of interludes of Iranian control) and the various dynasties that ruled Iran in the 17th-20th centuries. But when the Ottoman Empire collapsed and Iraq gained its independence, Iran and Iraq tried to settle the Shatt al-Arab issue diplomatically, with Iraq controlling the river and Iran paying tolls to use it.
This arrangement broke down starting in 1969, when Iran (easily the stronger power) simply refused to pay its tolls anymore. The Iraqi government didn’t respond with force, but it did break off diplomatic relations with Iran and began to encourage Arabs living in Iran’s Khuzestan province to think about revolting against Tehran (the Iranians, for their part, started to do the same thing with Iraq’s Kurds). After a couple of minor border conflicts, the 1975 Algiers Agreement was supposed to settle the issue, with Iraq mostly conceding control over the waterway in exchange for a return to normal relations between the two countries. The Iraqis came off worse in this deal, but relations did improve, and Hussein, who was the de facto ruler of Iraq and would become its formal head of state in 1979, saw the deal as a short-term arrangement that Iraq would “revisit” once it was strong enough. An oil boom that started in 1979 gave Iraq the money to build the kind of military it would need to commence with that revisiting.
Then the Islamic Revolution happened, and Iraqi Baʿathists’ irritation over the Shatt al-Arab mixed with a real fear that, once things were settled in Tehran, the revolution would roll over them next. Nowadays people who talk about this Iraqi fear of the revolution talk about it in terms of Iraq’s majority Shiʿa population, which was being repressed by the Sunni Baʿathists. And this is true to some extent–the first revolutionary elements in Iraq developed within its Shiʿa community, and Saddam long viewed the Shiʿa as a potential fifth column for Iran. But this framing ignores the fact that Ayatollah Khomeini’s message wasn’t initially conceived as a primarily sectarian thing. Yes, Iran was/is a mostly Shiʿa nation, and yes there were elements of Iran’s revolution that were specific to its Shiʿa society and religious establishment, but the vision of an Islamist revolt against corrupt, repressive secular autocrats (like, ahem, Saddam Hussein) was intended as a pan-Islamic idea, something that could happen anywhere and everywhere among Muslims of all stripes. So what really scared Saddam and company was not that Iraq’s Shiʿa would revolt, but that the entire country would.
Thus, and I don’t say this to excuse him in any way, Saddam did have reasons for wanting to go to war. He had only just taken power in his own right and wanted a big military victory to cement his stature. He wanted to stop the Islamic Revolution before it could spill out of Iran. He wanted control over the Shatt al-Arab. He even wanted Khuzestan and its oil. And now, finally, he had the money to go to war with Iran and potentially win. Iran, meanwhile, was doing what most revolutionary governments do right after they come to power: purging any senior elements of the previous regime. This meant that most of Iran’s most senior military officers were being “retired,” if they were lucky, or executed, if they were not, and those who could get the hell out of Iran were doing so. This change in relative military fortunes convinced Saddam that it was time to take Tehran on. All it took were a few skirmishes between revolution-minded Shiʿa militias and Iraqi security forces, starting the spring of 1980, and Saddam had his justification for war. Iraqi forces began shelling Iranian positions over the border and trading fire with Iranian troops.
I don’t want to rehash the entire war because we’d be here forever. So let’s just mention September 22’s results, which weren’t nearly what Iraq wanted and helped ensure that the war would be a long slog rather than a lightning Iraqi victory. Iraqi airstrikes, which were intended to eliminate Iran’s air force, really barely dinged it. Iran’s air force was able to quickly retaliate and deal serious damage to the Iraqis, more or less establishing air superiority. The Iraqi invasion, which began the next day, depended to some extent on the Arabs in Khuzestan revolting against Tehran and joining the Iraqi forces, which never happened, and Iran’s strength in the air was decisive in blunting the Iraqi offensive. Iraq turned to chemical weapons, and things settled in to a terrible status quo that lasted for nearly eight more years.