Mosul, scene of a major US-led offensive on Tuesday against Iraqi insurgents after a spate of deadly clashes, is an ancient and ethnically diverse city that has become a new front in the insurgency.
Car bombings and fighting have become all too frequent in Mosul and surrounding areas, which have gradually fallen into the sway of hardline Islamic groups since Saddam Hussein's regime was toppled in April 2003.
US troops were swarming into restive pockets on Tuesday to secure police stations and restore order, with bridges straddling the Tigris river closed and a night-time curfew in place.
Rebels overran a number of police stations last week, triggering clashes that left several dozen people dead, mostly insurgents.
US military commanders say the events in predominantly Sunni Muslim Mosul and other parts of the country may be a spillover from the massive assault launched last week on the insurgent hub of Fallujah.
But they insist they are still in control of the capital of Nineveh province, 370 kilometers (230 miles) north of Baghdad, at the tip of the so-called Sunni Triangle and scene of the worst violence since Saddam fell.
Mosul, whose name in Arabic means the link, is one of the most ethnically diverse cities in Iraq with Arabs, Syriac people, Armenians, Kurds, Turkmen, Jews, Christians, Muslims and Yazedis all calling the city home.
The area has been inhabited since 6000 BC and was chosen by the ancient Assyrians to build their glorious capitals of Ashur, Nimrud and Nineveh, whose ruins still dot the city and surrounding areas.
The city fell into the hands of the Persians, Romans and the Arabs and then Ottomans for almost seven centuries.
Its Nabi Yunis mosque is said to be the burial place of the reluctant prophet Jonah, who according to the bible fled to sea after turning down God's request to preach in Nineveh and was swallowed by a whale.
The area had been one of the most loyal pro-Saddam strongholds and it was a local tribal leader who sheltered his two feared sons Uday and Qusay in his Mosul mansion when they fled Baghdad.
But the same man, Nawaf Mohammed al-Zaidan, is believed to have tipped off the Americans on the whereabouts of the brothers, pocketing a 30-million-dollar US bounty that had been placed on both of their heads.
The two were killed in a fierce battle with US troops in July last year - an incident which left a strong impact on the local Sunni population - once Iraq's elite and now feeling increasingly marginalised since Saddam's fall.
Widespread looting and intercommunal fighting and killing swept through the city in April 2003 after thousands of Kurdish peshmerga fighters and special US forces seized the city during the US-led invasion of Iraq.
In recent months it has seen a rash of suicide car bombings and ambushes against US and Iraqi military convoys, Kurds, judges, government officials including the Nineveh provincial governor - anyone regarded by the insurgents as collaborating with US forces or the US-backed government.
Several Turkish truck drivers supplying goods to US military bases have been attacked or kidnapped in the area. The north-south roads running through Mosul have become so treacherous that many truckers have stopped coming to Iraq altogether or travel only in US protected convoys.
Iraq's Defence Minister Hazem Shaalan has charged that Mosul and surrounding areas are safe havens for militants from Syria, which is about 180 kilometers (110 miles) to the west.
Sunni Muslims in Mosul, together with the minority Turkmen community, fear Kurdish calls for an expanded autonomous region in districts immediately bordering the northern metropolis, a city of about 1.5 million people.
For their part, the Kurds regard the Turkmen, who share the same ethnic origins as the Turks, as Ankara's fifth column in an area once part of the Ottoman Empire. Turkey has been apprehensive of increased Kurdish power in Iraq fearing that would stir up its own Kurdish population.
Iraq's interim president Ghazi al-Yawar hails from Mosul, and his fellow Al-Shamari tribesmen who are dominant in the area are among the most powerful in all of the country.
The US military maintains a force of about 10,000, including some Australians and Albanians, in Mosul and surrounding areas with a command base at one former palace.