Mosul—Iraq's third-largest city, urbane center of the nation's northern regions, built on the remains of the once-great Assyrian empire, made free through the sacrifices of thousands of Iraqis, Kurds, and Americans—fell to a gang of Islamist insurgents Monday.

Fighters for the Islamic State of Iraq and Syria, bastard progeny of Al Qaeda in Iraq, advanced on the city over the weekend, as they had done successfully in Fallujah earlier this year. As the ISIS jihadis approached, soldiers of Iraq's U.S.-trained army abandoned their posts and weapons and fled, leaving the southern part of the city and nearly 2 million local inhabitants free for the taking.

Mosul is perhaps the most important site yet contested by the multinational brigade of jihadis, for whom it is a financial and logistical hub for fighting both Syrian dictator Bashar al-Assad and the Iraqi coalition government in Baghdad, led by Prime Minister Nuri al-Maliki. For years through the Iraq war, the oil-rich city alternated between bloody battles and relative peace, thanks in part to painstaking efforts by the American forces whose presence had destabilized it in the first place.

Maliki has urged Iraq's Parliament to declare a state of emergency, and he's likely to ask the international community for help. Good luck with that:

The capture of Mosul demonstrates that the insurgents now have the capacity to seize strategically vital territory, positioning them to threaten other important areas of Iraq, said Charles Lister of the Doha Brookings Center based in Qatar.

It also raises questions about the continued utility of sending U.S. military support to Maliki, whose security forces seem simply to have crumbled. Maliki is urging the United States to deliver more advanced weaponry, but ISIS fighters have already been seen riding round in U.S.-supplied Humvees in other areas they control, and much of the weaponry captured in this latest battle is likely to be American, Lister said.

ISIS—which fused elements of Al Qaeda in Iraq with anti-Assad Syrian fighters, but has broken publicly with global Al Qaeda leader Ayman al-Zawahiri in recent months—gleefully tweeted out photos of its fighters frolicking on those American Humvees:

Meanwhile, the city's civilian population is making for the exits any way it can.