The outbreak of full-scale war between Iraq and Iran seems to have expunged from view the immediate cause of the war, not to speak of drowning out Iraq's claim of limited objectives. In fact, the longstanding dispute over control of Shatt-al-Arab -- the shallow, muddy waterway that unites the historic Tigris and Euphrates Rivers from their confluence at al-Qurnah to the head of the Persian Gulf -- is what triggered the Iraqi assault. After both sides run out of military hardware, its status will be the prime issue at the bargaining table.

Shatt-al-Arab, whose lower 50 miles constitute a disputed border, is a vital economic and strategic artery for both Iraq and Iran. Except for the railhead of Umm Qasr on the Gulf, it is Iraq's lifeline to the sea, enabling medium-draft vessels to bring cargoes upriver to Basra and providing an exit route for those petroleum exports that do not go by pipeline. For Iran, the waterway is equally important. It provides direct access by water to the oil ports of Khorramshahr and Abadan for machinery and equipment, and a surface route for trans-shipment of part of the three million barrels of oil a day that used to find their way to the supertankers lying offshore in the Gulf.

Far from being a mere pretext in a larger quarrel, the dispute over control of Shatt-al-Arab in its present form dates back for well over a century. As late as 1700, Shatt-al-Arab was an inland waterway, albeit a border one, of the Turkish Ottoman Empire, with the Arabic-speaking tribes on both banks claimed by Turkey. Subsequent reassertion of Persian control over what is now Khuzestan led to repeated border conflict, and in 1847 a boundary treaty between the Turkish and Persian empires was negotiated that provisionally fixed the border on the east bank of the waterway, giving Persian vessels freedom of navigation along its entire length. An Anglo-Russian boundary commission was later appointed to define the exact boundary and to work out arrangements for equitable apportionment of river rights. In 1913, after innumerable delays, these efforts led to agreement between the two empires on an amendment to the 1847 treaty providing for new delimitation a year later.

The 1913 agreement and subsequent delimitation left "the river and all islands therein" to Turkey, with the exception of two strips of about four miles each opposite the ports of Khorramshahr and Abadan. There the boundary was shifted to the mid-channel line, or thalweg , a concept in international law that so defines the demarcation in international rivers between territories of adjacent states. Under the protocol, Persia continued to retain navigation rights up and down the waterway, though not, of course, control over navigation. The newly created state of Iraq, which after World War I emerged from the wreck of the Ottoman Empire, succeeded to the rights conferred by the 1847 and 1913 agreements, at least in theory.

During the 1920s and '30s, these agreements came under persistant attack by Iran, which took to assailing their validity on technical grounds and asserting that the thalweg principle should apply to the full length of the waterway. Iraq took the treaties to the League of Nations Council and the Permanent Court of International Justice in a fruitless effort to obtain reaffirmation of its rights, but Persia managed to frustrate these attempts. Nevertheless, Iraq continued to control pilotage, dredging and navigation, though not without recurrent quarrels with Iran over the latter's failure to remit its share of port dues for maintenance.

After World War II, the transformation of the whole region, and especially the Iranian east bank, into an oil-producing complex of massive dimensions dramatically raised the stakes. On the other hand, the increase in waterway traffic to Basra, Khorramshahr and Abadan, coupled with the need to deepen the channel and keep it free from the 35 million tons of silt that flows annually downstream from the mother rivers, placed an extraordinary burden of responsibility and expense on Iraq. On the other, Iran chafed under what it perceived to be hostile Iraqi administrative control of the access routes to its two key oil ports, repeatedly trying to convert a provision in the 1937 treaty for a joint river commission into a mandate for joint control.

When in the late '60s the shah's military power -- not to mention his overweening vanity -- began to peak, the constraints on traffic imposed by what he considered an irresponsible, radical and, above all, inferior neighbor began to seem insupportable. In April 1969, the shah denounced the 1937 treaty and declared mid-channel to be the boundary. In 1975, he followed up by maneuvering Iraq into an agreement signed in Algiers that confirmed the thalweg boundary for the entire waterway, the trade-off being a pledge to withhold support from the Kurdish rebels. It fell to Iraq's current leader, Saddam Hussein, to sign the Algiers agreement in his capacity as vice premier. On Sept. 17, 1980, with Iran in chaos, Saddam denounced the Algiers agreement and five days later launched the attack.

When the guns fall silent, Shatt-al-Arab will continue to be crucial to the economies of both Iraq and Iran, not to mention the energy needs of the rest of the world. Unless Iraq proposes to occupy Iran's most priceless asset, the Abadan oil region -- thereby creating a territorial grievance that will make the West Bank seem like a neutral panic ground -- it will sooner or later have to renegotiate the status of the waterway, and on terms that will provide unrestricted entry to the two Iranian oil ports. Iraq has history and the sanction of treaty law on its side, at least up to 1975. Iran has prescriptive rights of navigation and access, coupled with an obligation to pay its fair share of dredging and maintenance.

Shatt-al-Arab is a multifaceted system that needs to be administered as such. It is not too soon to initiate a diplomatic effort by interested neutrals, preferably under Islamic auspices, to lay the groundwork for a new legal regime that will satisfy both countries and the global community.