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A half-million people fled the Iraqi city of Mosul after it fell to militants from an Al Qaeda offshoot on Tuesday. The group, Islamic State of Iraq and the Levant (ISIS), now appears to be fighting for control of the city of Baiji, located about halfway between Mosul and Baghdad, the capital.

The Geneva-based International Organization for Migration reports that an estimated 500,000 people have been forced to flee areas in and around the city amid fighting between ISIS and Iraqi security forces. The organization adds that the city is facing shortages of food, fuel, and water – the last because the city's main water station was destroyed by bombing – and that the main medical center, which contains four hospitals, is largely unreachable due to fighting.

The IOM's report underscores the humanitarian and political crisis that Mosul's fall has precipitated for Baghdad. Mosul is one of Iraq's biggest cities – it has been described as both the second- and third-largest – and one of its most diverse, as home to Sunnis, Shiites, and Kurds. It is also the commercial hub of northern Iraq and a cornerstone of US post-war stability efforts.

But despite its importance and the known presence of insurgent groups in the city, Mosul fell quickly to ISIS forces; their assault, which began on Friday, was a rout by Tuesday, reports Reuters. Many Iraqi troops fled in the face of ISIS' assault.

Al Jazeera America reports that ISIS has some 1,300 fighters in and around Mosul, and their ranks may have been bolstered with the release of 2,400 prisoners from Mosul's jails.

ISIS now appears to be focusing its attention further south, on the oil town of Baiji. Al Jazeera reports that ISIS negotiated the retreat of the 250 security officers in the town's main refinery, who left when their safety was guaranteed. ISIS subsequently advanced into the town, torching the police and fire departments and sending thousands of residents fleeing.

º£½Ç´óÉñ's Dan Murphy writes that "the ISIS victory is a signature moment – evidence that ISIS can't be dismissed as merely a ragtag group of Sunni insurgents who may be able to hold their own in Syria, but would be unlikely to take and hold ground from Iraqi forces."

The Telegraph's Richard Spencer argues that ISIS' success in Mosul can be attributed in part to the effectiveness of its "advertising" in Syria, where it has been waging war against President Bashar al-Assad's regime, dominated by members of the Shiite Alawite sect.

Mr. Spencer adds that in Iraq, ISIS has been careful not to alienate local populations, seeking to capitalize on Sunni hostility toward the Shiite-dominated central government, which has taken steps to cut Sunnis out of the political process.

Spencer notes that the group's success seizing the Iraqi cities of Fallujah and Ramadi a few months ago would not have been possible without local support, implying that ISIS would have needed similar backing in Mosul.