Where does one country's river become another's? The question sounds academic until armies start shooting over the answer. The Shatt al-Arab, formed by the confluence of the Tigris and Euphrates rivers, flows roughly 200 kilometers to the Persian Gulf along a stretch of the Iran-Iraq border. For decades, this murky waterway generated more diplomatic crises per kilometer than almost any other body of water on Earth. In 1974, the argument turned lethal. Over the course of a year, Iranian and Iraqi forces fought a series of cross-border clashes that left more than a thousand people dead, drew in Kurdish rebels, American intelligence agencies, and Israeli arms suppliers, and ended with a deal signed in Algiers that would hold for exactly five years before collapsing into one of the longest conventional wars of the twentieth century.
The roots of the conflict reached back generations. Under Ottoman rule, the entire waterway had belonged to what would become Iraq. But Iran had contested this arrangement since the early twentieth century, arguing that international law required the border to follow the thalweg, the deepest channel of the river, rather than hugging the Iranian bank. In 1937, the two countries signed a boundary treaty that placed the border along Iran's eastern shore, with one exception: a four-mile zone near the port city of Abadan, where the line ran along the thalweg. Iran accepted the lopsided deal reluctantly. When Iraq's Hashemite monarchy fell in the 1958 coup, and then the Ba'ath Party seized power in a second coup in 1968, Iran saw an opportunity to renegotiate. In 1969, after the new Iraqi government refused to discuss a revised treaty, Iran simply withdrew from the 1937 agreement altogether. The border was now, legally and practically, an open question.
The actual fighting grew out of a web of proxy conflicts. Iran had been arming Iraqi Kurdish rebels fighting for independence from Baghdad, working alongside the United States and Israel. A 1973 raid on the Iraqi embassy in Pakistan exposed covert Iraqi support for militants in Balochistan, further poisoning relations between the two governments. By March 1974, these tensions spilled into open combat along the Shatt al-Arab. Border skirmishes escalated through the spring and summer. In 1975, Iraq launched a tank-led incursion across the Iranian border, a gamble that backfired badly. Iran possessed the world's fifth-largest military at the time. Its air force made short work of the Iraqi armored columns, and its continued support for Kurdish insurgents kept Baghdad fighting on two fronts. Over a thousand soldiers and civilians died in the year-long confrontation, and Iraq gained nothing from it.
Facing military stalemate and a Kurdish rebellion it could not suppress, Iraq chose diplomacy. In March 1975, Saddam Hussein and the Shah of Iran met in Algiers, mediated by Algerian President Houari Boumediene. The resulting agreement split the difference on the waterway: Iraq ceded roughly half of the disputed border area, accepting the thalweg line Iran had demanded for decades. In exchange, Iran cut off its support for Iraqi Kurdish rebels. The effect was immediate and devastating for the Kurds, who saw their insurgency collapse within weeks once Iranian weapons and sanctuary vanished. The agreement held through the rest of the 1970s. But it was a deal made under duress, and the man who had signed it on Iraq's behalf never forgot the humiliation.
The 1979 Iranian Revolution changed everything. The Shah who had brokered the Algiers Agreement was overthrown, replaced by Ayatollah Ruhollah Khomeini's Islamic Republic. Iraq's Ba'ath Party feared Khomeini would incite Iraq's Shia-majority population to revolt. When Iranian forces shelled Iraqi border posts on September 4, 1980, Saddam Hussein saw his chance. On September 17, he formally abrogated the Algiers Agreement, declaring that Iran had already violated its terms. Five days later, the Iraqi military launched a massive invasion of Iran. The Iran-Iraq War that followed would last eight years, kill hundreds of thousands of people, and devastate both nations. The 1974-1975 conflict, a year of skirmishes over a river border, had been the rehearsal. The waterway that two countries could not agree how to share became the fault line along which an entire region fractured.
The Shatt al-Arab waterway is visible from altitude as a wide, brown river running southeast to the Persian Gulf, located at approximately 30.41°N, 48.15°E. The confluence of the Tigris and Euphrates near Basra is a distinctive landmark. Basra International Airport (ORMM) lies nearby to the west. Abadan Airport (OIAG) sits on the Iranian side. The flat terrain and the broad waterway make the Iran-Iraq border easy to trace from the air, with date palm groves lining both banks.