On Tuesday, insurgents representing a group which calls itself the Islamic State of Iraq and Syria stormed and captured Mosul, the second-largest city in Iraq. The armed rebel group had recently been disavowed by Al-Qaeda leaders, who disagreed with the extreme brutality of ISIS’ tactics. The Washington Post is reporting that, as ISIS fighters charged into the city from the west, many US-trained soldiers and police officers representing the Iraqi government dropped their weapons, stripped off their uniforms, and fled their guard posts, allowing ISIS to seize the entire city and several bases full of weapons, most of which were likely provided by the United States.

According to International Business Times, up to 500,000 residents of Mosul fled in advance of the attack. The city’s population prior to these evacuations was estimated at around 1.5 million people, making this a significant victory for the Sunni insurgency. ISIS also captured Fallujah in January. Iraqi Prime Minister Nuri al-Maliki requested that Parliament declare a state of emergency and called on Iraqis to take up arms and fight back against the insurgents, which may have signaled to ISIS that the government lacks the manpower and resources to defend itself against the group’s advances.

Amid reports that the US is sending F-16s to Iraq, ISIS fighters seized a massive stockpile of weapons and vehicles, which likely included munitions supplied by the United States. The infamous Chechen jihadi fighter Omar al-Shishani was depicted in a photograph on Twitter examining a US-provided Humvee which may have been seized during the attack. The loss of US-supplied weapons to insurgents raises questions as to whether or not the Iraqi government has the ability to keep advanced American weaponry from falling into the wrong hands.

The Islamic State of Iraq and Syria, also referred to as the Islamic State in Iraq and the Levant, claims to be an independent state representing Iraq and Syria, but is unrecognized by international bodies and is considered a rebel group by the Iraqi government. ISIS now controls a significant portion of land in the region, encompassing sections of eastern Syria and western Iraq, and plans to capture the Levant region in the future. Prime Minister Maliki has announced that security forces will be mobilized to combat the invasion.

The sudden seizure of such a large city has shocked observers, causing experts to reassess the strength of ISIS’ fighters. Also, the fact that government forces fled in advance of the attack calls into question the loyalty of Maliki’s soldiers and police officers, some of whom might have had sympathies for the insurgents.