FACTBOX - Key facts on Basra, Iraq's second city

(Reuters) - Heavy fighting erupted on Tuesday in the southern oil city of Basra where Iraqi security forces launched a major operation at dawn against powerful militias, military officials and witnesses said.

Here are some details about Basra:

* GEOGRAPHY:

-- Basra is the main port of Iraq and situated on the western bank of the Shatt al-Arab, the waterway formed by the union of the Tigris and Euphrates rivers. Basra is 550 km (340 miles) south of the Iraqi capital, Baghdad.

* SOME HISTORY:

-- Basra was founded as a military encampment by the second caliph, Umar I, in AD 638 about 8 miles (13 km) from the modern town of Az-Zubayr. The first architecturally significant mosque in Islam was constructed there in 665.

-- By the 14th century, neglect and the Mongol invasions left little of the original Basra standing, and by the turn of the 16th century it was relocated at the site of the ancient Al-Ubullah, a few miles upstream.

-- In the 17th and 18th centuries, English, Dutch, and Portuguese traders were established there, and Basra developed considerably during the 19th century as a trans-shipment point for river traffic to Baghdad. In 1914 the construction of a modern harbour began at Basra, which previously had had no wharves.

* WORLD WAR TO GULF WAR: