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The waters where Iranian navy forces seized 15 British sailors and marines are near an area long contested by Iran and Iraq, just outside the mouth of a muddy, 200 kilometer (125-mile)-long channel that separates the Arab and Persian worlds.
The neighbors don't even agree what to call it.
For the Iranians, it is Arvandrud, or Arvand River. To the Iraqis, it is the Shatt al-Arab, or Arab coastline.
Whatever the name, the waterway is formed by the confluence of the Tigris and Euphrates rivers, which join at the Iraqi town of Qurnah and flow into the Persian Gulf, providing Iraq's only outlet to the sea.
Iranians and Iraqis have been squabbling over the waterway since before the creation of modern Iran and Iraq.
In 1639, the Persians and Ottomans signed an agreement dividing the territory, but without a careful land survey. Bickering continued for centuries, with Iraq claiming the entire waterway right up to the Iranian shoreline.
In 1975, however, the two countries signed a treaty recognizing the middle of the waterway as the international boundary. Five years later, Saddam Hussein tore up the treaty and invaded Iran, unleashing a devastating eight-year war.
Much of the fighting centered around the waterway. When the Iranians captured the Faw Peninsula on the southern end of the channel in 1987, Iraq lost its shipping lanes and had to turn to Kuwait and Jordan for help in exporting its products.
A United Nations peacekeeping force was stationed along the Iran-Iraq border after the war ended, from 1988-1991, with naval personnel in the waterway.
British forces overran the area in the early days of the 2003 invasion that toppled Saddam and quickly reopened the port of Umm Qasr. Virtually all of Iraq's oil is exported through an oil terminal near the mouth of the waterway.
In June 2004, six British marines and two sailors were seized by Iran there. They were presented blindfolded on Iranian television and admitted entering Iranian waters illegally, then released unharmed after three days.
Today, the UN says it is not involved in any border disputes between the countries.
"It is up to each country to decide their borders," said Farhan Haq, a UN spokesman. "The United Nations does not draw borders. What the recognized borders are in that waterway is the decision of Iran and Iraq."
But Teheran and the new Iraqi government have never signed a treaty to replace the Shah of Iran's agreement with Saddam, meaning control of the waterway remains a matter of dispute, said Lawrence G. Potter, an associate professor of international affairs at Columbia University.
"The problem is that nobody knows where the border is," Potter said. "The British might have thought they were on their side, the Iranians might have thought they were on their side."
For the Iranians, who have a much longer coastline, exercising their claims to the area is both emotional and strategic. The Iranian cities of Abadan and Khorramshahr lie along the waterway.
Michael O'Hanlon, senior fellow of foreign policy studies at the Brookings Institution, said the Iranian interest in defending its claims explains the move against the British.
"The only reason they would do what they've done is to show that they feel strongly about those waters," he said. "This is the sort of thing they will want to ratchet down very quickly."
Shibley Telhami, a senior fellow at the Brookings Institution, said that he doubted that the Iranians would risk harming their relationship with the Iraqi government over the waterways.
"There's always been a dispute about where the line is, but it returned to status quo since the end of the war in 1988," said Telhami. "They have a good relationship with the Iraqi government right now, so it would be surprising if they wanted to open border issues with them at this point."
With tensions high over Iran's refusal to halt uranium enrichment, and recent US detentions of Iranians in Iraq, Potter said, Iran may have found a way to assert itself internationally without provoking serious repercussions.