Like many cities in Iraq, Mosul's history is ancient. It got a mention by the ancient Greeks as far back as 401 B.C. It was a center of manufacturing and trade in the Middle Ages. It plays an important role in the oil trade today.
But the most important role the city plays today is as the Iraqi capital of the militant group Islamic State. Out of a population of between 1.5 million and 2 million, 4,000 to 8,000 are armed extremists.
They now face a combined military force in the tens of thousands, backed up by some of the world's great military powers, including the United States. The Iraqi Army and Special Forces are joined by the Kurdish peshmerga and Shi'ite militias that have reportedly been trained and at least partially equipped by Iran. U.S. Special Forces and advisors are also on the ground, though their numbers and exact role is less clear.
A Pentagon-led coalition is also lending support from the air, dropping bombs and spotting targets. In all, some 66 countries are participating in the fight against Islamic State, according to the U.S. Department of Defense. In addition to Western nations like the United States, France, Britain and Denmark, Saudi Arabia and other Gulf states are counted as members.
Expectations everywhere are that Mosul will fall, though opinions differ on how bitter the fight will be and how long it will take. There is broad concern over the potential humanitarian crisis as people in the city are crushed between militants and militaries. How many will flee the fighting and become refugees in a region flooded by them?
And who will rule the city when the fighting ends? The official Iraqi government, Shi'ite militias and the Kurds have all shed each others' blood in the past. Mosul was once a multicultural city, but Saddam Hussein tried to make it an Arab one. After Saddam's fall, Arabs were forced from their homes.
Now, how will the city be governed? And what will be left of it? Peter Van Buren, a diplomat with more than two decades of experience, is not optimistic.