Five years earlier, in March 1975, a deal signed in Algiers between the Shah of Iran and Saddam Hussein -- then Iraq's vice president -- had tried to settle the argument.

The Algiers accord ruled that their border ran along the centre of the Shatt al-Arab, a 200 kilometre (125 mile) long river formed by the meeting of the Tigris and the Euphrates, and that flows into the Gulf.

But in April 1980, Baghdad accused Tehran -- now the Islamic Republic of Iran, after the 1979 toppling of the Shah -- of plotting attacks.

Iraq called for the evacuation of three strategic islands in the Strait of Hormuz, claimed by both Iran and the United Arab Emirates.

On September 17, Baghdad said the Algiers accord was null and void.

It demanded all of the Shatt al-Arab.

On September 22, Saddam Hussein sent soldiers into Iran.

His air force bombed airports -- including that of Iran's capital Tehran -- as well as military targets, and oil industry infrastructure.

The oil refinery of Abadan, one of the biggest in Iran, was shut down.

In the first weeks, Iraqi forces met little resistance.

They seized the towns of Qasr-e Shirin and Mehran, and captured Iran's southwestern port of Khorramshahr, where the Shatt al-Arab meets the sea.

Saudi Arabia and Kuwait swiftly offered support to Baghdad.

Arab nations -- including the rich Gulf countries dominated by Sunni Muslim leaders -- gave billions of dollars to Iraq.

They saw Saddam Hussein as a bulwark against the Islamic Revolution of Iran's Ayatollah Ruhollah Khomeini, a Shiite Muslim.

Western countries, alarmed at the Iranian clerics who overthrew their old ally in Tehran, the Shah, sold weapons to Iraq.

In March 1982, Iran launched a major counter-attack in the southwestern oil province of Khuzestan, taking back its port of Khorramshahr.

Baghdad announced a ceasefire and pulled back troops.

But Tehran rejected the ceasefire.

Iran continues the fight, bombarding the major Iraqi city of Basra, and in July, begins an offensive on the southern front.

Iraq in August blockades the main oil terminal on Kharg Island, just off Iran's coast.

From April 1984, the two sides engage in a "war of the cities".

Some 30 cities on both sides are battered by missile attacks.

In 1984, Iran accuses Iraq of using chemical weapons on its soldiers in battles in the marshes of Majnoon.

The UN confirms the accusations.

Baghdad strengthens its maritime blockade of Iran.

Tehran responds by attacking oil tankers loading up at Gulf ports of Iraq's allies.

In 1986, as Iraq launches raids on Kharg, Iran's army crosses the Shatt al-Arab and seizes the Faw Peninsula, in Iraq's south east.

In June 1987, Iraq drops poison gas canisters on the Iranian town of Sardasht.

In March 1988, Baghdad is again accused of using chemical weapons -- this time against its own population, in the Iraqi town of Halabja.

The town was controlled by Kurdish fighters, backed by Iran.

Some 5,000 were killed in Halabja by the gas attacks.

From April 1988, Iraq launches another offensive, taking back Faw, Majnoon and the southern region of Shalamsheh.

Iran is pushed back across the Shatt al-Arab.

On July 18, Ayatollah Khomeini accepts a UN Security Council resolution -- approved a year earlier and already accepted by Iraq -- to stop the fighting.

"This decision was for me even more painful than taking poison, (but) I accepted that that was what God had decided," Ayatollah Khomeini said.

While the exact number of those killed in the war is not known, at least 650,000 died, roughly two-thirds of them Iranians, according to French historian Pierre Razoux.

However, it takes two more years before the Algiers accord is restored, in August 1990, for Baghdad to withdraw troops from Iran, and for an exchange of prisoners of war.