On Tuesday, al-Qaeda-linked fighters belonging to Islamic State of Iraq and Syria (ISIS), a powerful extremist militia, captured the key Iraqi city of Mosul. (The Washington Post's Liz Sly recounts the city's fall here and explains ISIS's alarming rise here.) With his forces beating a hasty retreat, Iraq's beleaguered prime minister, Nouri al-Maliki, called on parliament to institute a state of emergency in the country. Reports indicate ISIS forces may be prepared to advance on other major cities nearby.

In recent months, the group, which has gained strength amid the Syrian civil war, made inroads through Iraq's Sunni heartland, overrunning the towns of Ramadi and Fallujah in January. But Mosul is a far bigger prize: the oil-rich capital of Nineveh province is Iraq's second city, a northern center that's home to a microcosm of the country's mixed population.

The militants have already commenced looting the city's arsenals, weaponry that in many cases was provided by the United States.  Hundreds of thousands of civilians have already fled the city, which has a greater metropolitan population close to two million. The International Organization of Migration's Iraq office estimates that half a million people from Mosul have already been displaced by a few days of violence.

Mosul is steeped in history -- and no stranger to conquest. The modern-day city sits across the Tigris river from the ruins of ancient Nineveh, a great Mesopotamian center and crossroads for trade whose origins may date back 5,000 years. Mosul was for centuries home to an array of communities: Arabs, KurdsSyriac Christians, Turkmen and even a small population of Jews.

Its strategic importance remained well into the medieval era. Successive dynasties and marauding Turkic tribes besieged and overran the town. Mongols captured it in 1262; later, the Ottomans would seize it and make it into a prominent administrative hub of the region.

Following the collapse of the Ottoman Empire after World War I, the new Turkish state squabbled with the British over possession of the city and grudgingly ceded its claim to the Kingdom of Iraq in 1926 in return then for a fraction of its annual oil revenue.

Under the dictatorship of Saddam Hussein's Arab nationalist Ba'aath Party, the government aggressively Arabized the city. After his fall in 2003 -- Hussein's two sons, Uday and Qusay, were cornered and killed by U.S. forces in a Mosul hideaway -- the city was for a brief spell considered a model of American counterinsurgency and a feather in the cap of the U.S. official tasked to keep the peace there, Gen. David Petraeus, then the commander of 101st Airborne. But, like much else in Iraq during the U.S. occupation, the situation unraveled and the city was witness to routine bombings, disappearances and murders.

In recent years, Iraq's autonomous Kurdish government has asserted its own claims upon Mosul and ethnic tensions between Arabs and Kurds have flared frequently into violence. There have been fears of a full-blown civil war between Kurdish regional forces and the Iraqi army. A 2009 report from the International Crisis Group explained the hostilities.

Ethnic relations in [Nineveh province] have a chequered history. The struggle between Arab and Kurdish nationalisms has been especially acute, notably in the capital, Mosul, home to deeply rooted Arabist feelings. The Kurds have paid a heavy price. The state has made aggressive attempts to contain or suppress their national aspirations. The Baathist regime in particular engaged in forced displacement and discriminatory resource distribution. Kurds saw a chance for redress in 2003 and seized it, launching an offensive to rewind the clock and undo the effect of past practices. This too had a cost. Operating largely in an ad hoc manner, without due process and by dint of force, they took control of several districts, including many towns and villages, seeking to incorporate them into the Kurdistan region and, largely thanks to the Sunni Arab boycott of the 2005 provincial elections, they established political dominance in the governorate.

Now, leaders both in Iraqi Kurdistan and Baghdad must be bracing for a fight with a new threat, one which no longer can be ignored. Arab journalists tweeted out rough maps of the forces arrayed in Mosul.

The battle lines are drawn; the signs are not encouraging. ISIS fighters are clearly well-trained, well-equipped and capable of thwarting the disorganized Iraq army. "We can't beat them. We can't. They are well trained in street fighting and we're not. We need a whole army to drive them out of Mosul," an Iraq army officer told Reuters. "They're like ghosts: they appear, strike and disappear in seconds."