Mosul became a new focus for attacks on US forces and their Iraqi allies after the fall of the rebel bastion of Falluja in November 2004.
US troops have enemies among the Sunni Arab community in Mosul
Rich in oil and history, it is Iraq's third-largest city, marking the volatile border between the country's Kurdish and Sunni Arab populations.
More than half of Mosul's nearly 2 million people are Sunni Arabs, some of them brought to the area by Saddam Hussein in an effort to dilute Kurdish dominance.
Kurds, who control the region to the north, account for about 500,000 of the city's inhabitants. Ethnic Turks, Yezidis and Christians make up the remainder.
US-led forces battling to retake Falluja in November were diverted to Mosul to quell a short-lived uprising, during which rebels seized several police stations and made off with weapons and uniforms.
The insurgents melted away as the US regiment's distinctive eight-wheel Stryker tanks returned to Mosul's streets.
In the weeks that followed, the corpses of captured Iraqi government soldiers began appearing on the roads.
December's attack on a dining hall at the US military base in Mosul killed more than 20 people.
James Reynolds, a BBC correspondent, says the military was aware its dining hall was vulnerable to attack and was building a stronger structure nearby.
Mosul, like Falluja, had a tradition of supplying officers for the army and intelligence services of Saddam Hussein.
A boy injured after US troops fired on a protest in April 2003
According to Charles Glass, who reported for the London Review of Books from Mosul, there are signs the city's anti-American insurgency is being marshalled by shadowy figures from the former dictator's Baath party.
A reporter in the city told the BBC News website in November that several Syrians were among the fighters captured by US forces.
Syria has repeatedly been accused by Baghdad's US-appointed leadership of offering a safe haven to fleeing Baathists and of sending insurgents into Iraq - charges it has denied.
Until the latest unrest, Mosul was largely known as the city that hid Saddam Hussein's sons, Uday and Qusay, before US forces stormed their safe haven, killing them both.
A demonstration against the US presence on 15 April 2003 led to several Iraqis being killed by US forces.
Mosul was founded by the Assyrians and the region around it, which includes the historic city of Nineveh, is thought to have been inhabited for at least 8,000 years.
An important staging post on trade routes linking Asia and Europe, Mosul was passed from empire to empire - the Romans held it briefly, followed by the Muslim Umayyad dynasty, the Abbasids and the Ottomans.
Its famous exports include marble and the fabric to which it probably gave its name - muslin.
Britain took charge of the city after the World War I, eventually making it part of the newly-crafted kingdom of Iraq.
British forces returned to the area after the first Gulf war, joining the US in policing Mosul's skies to enforce the no-fly zones imposed on Saddam Hussein.
Mosul's proximity to major oil reserves and its unstable ethnic mix have led some to fear it will become the powder-keg that ignites civil conflict across the region.