After four days of hard fighting, the Islamic State of Iraq and Syria rebel group has captured Mosul, a city of 1.5 million in Nineweh province in northern Iraq. ISIS celebrated its victory by circulating a letter, apparently penned by the Nineweh’s governor, declaring a full retreat of the Iraqi armed forces.

ISIS, which the United States has designated a terrorist group, has begun an major operation to capture key Iraqi cities. The Islamic group has dubbed the offensive “Operation Entering the Gates.”

The first attack targeted Samarrah just north of Baghdad. The city is home to two shrines which have immense value to Iraq’s Shia majority. ISIS is a Sunni group.

The attack was a classic ISIS operation. While the main front line was in the city’s west, suicide bombers in pickups blew up two check points in the east and opened a path for a force of armed technical trucks.

An Iraqi Mi-35 targets an ISIS technical in Mosul. Afagh TV capture

ISIS flanked the Iraqi forces. Seventy members of the Iraqi special forces held their position until air support arrived in the form of powerful Mi-35 gunship helicopters guided by OH-58 and Bell 407 scouts.

The Iraqi helicopters targeted ISIS supply lines as well as front-line units. ISIS field commander Abu Bakr Al Iraqi died in the assault. The Islamists fled, harassed the whole way by the Mi-35s and scouts.

The battle in Mosul was different. ISIS had attacked army checkpoints in and around the city for months. In late April, Islamists ambushed an Iraqi M-1A1 tank on Mosul’s outskirts. A Kornet missile set the heavy vehicle ablaze. The Islamists apparently captured the Russian-made Kornet from a Syrian army depot.

The rebels launched the main assault on Mosul on June 7, targeting a key government checkpoint protected by heavy concrete walls and supported by U.S.-made M-113 armed personnel carriers and M-1A1 tanks.

One M-1 tank was an easy target. The poorly-trained crew had left the hatches open. A Somali Islamist approached the tank and threw in a grenade that cooked off the vehicle’s ammunition. An Iraqi sniper promptly shot the Somali fighter.

The M-1 explodes after the attack by the Somali jihadist, visible at right moments before a sniper shot him. Al Anbar News photo

With technicals providing covering fire, one by one ISIS fighters took out the Iraqi vehicles, tossing grenades into open hatches. By noon, other ISIS elements were attacking checkpoints all around the city, firing RPG-7 rockets to set ablaze vehicles including American-supplied MRAPs.

What’s perhaps most astonishing is that many people in Mosul support ISIS. The Syria war has elicited sympathy for the Islamists’ cause among Iraq’s Sunni minority. Iraqi prime minister Nouri Al Malaki, a Shia and ally of Iran, has supported the Syrian regime and even allowed Iraqi Shia militia to enter Syria to aid the Syrian government.

At the first signs of battle in Mosul, Sunni tribesmen and the city’s big Sunni families formed small volunteer battalions and attacked army checkpoints from inside the city.

Iraqi MRAPs burn in Mosul. ISIS photo

On June 9, the situation escalated from alarming to disastrous. Iraqi army Lt. Gen. Ali Gheidan traveled to Mosul to hold talks with local elders and to calm the population. But on Monday night, the talks collapsed and the governor announced the retreat.

Iraqi warplanes flew top cover as government troops fled Mosul. Many Iraqi officers put on civilian clothes in order to blend in with refugees leaving the city.

The next ISIS attack could target Tikrit, just 20 miles from Baghdad. The Iraqi army possesses the manpower and weaponry to defeat the Islamists’ advance, but rebellion by everyday Sunnis complicates Baghdad’s battle plans.

Iranian Maj. Gen. Qasem Souleymani, head of Iran’s notorious Qods force, has gone to Iraq to confer with the country’s Shia militias—and convince them to support the Iraqi government. Iraq’s Kurdish regional government has put its feared peshmerga militia on full alert on the roads leading out of Mosul.

Iranian president Hassan Rouhani has met with Turkish prime minister Recep Erdoğan to discuss possible intervention in Iraq. The deputy prime minister of Iraq has asked the United States to intervene.

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