How and why the Iran-Iraq war started

Origin of war: On Sept. 22, 1980, Iraq launched an air attack on 11 bases in Iran's Khuzestan Province.

At issue was the Shatt al Arab waterway, which links the Gulf to major oil ports of both countries. The two nations had long been at odds over control of the waterway and three islands in the Gulf: Greater and Lesser Tunb, and Abu Musa.

The dispute was supposedly resolved by the 1975 Algiers pact, which divided the waterway down the middle.

But Iraq abrogated the accord on Sept. 17, 1980, charging Iran with noncompliance. Iraq then demanded complete control of the waterway, and began the long war against its neighbor.

Possible reasons:

The most widely held view is that Iraq felt insecure about the 1979 revolution in Iran, where Ayatollah Khomeini planned to ''export Islamic revolution.'' Iran's military, armed with US weapons obtained by the Shah, was regrouping. Iraq, it is believed, thought it could successfully strike during this period.

* Iraq may have hoped to gain control of Iran's Arab-populated southwestern region, where most Iranian oil lies, thereby increasing Iraq's influence in OPEC.

* Iraq may have been making a bid for leadership in the Arab world.

* Iraq was angered by Iranian support for Kurdish rebels in Iraq.

* Historical Arab-Persian animosity fanned tensions.