Seen from above, how one of the largest airstrikes against ISIS unfolded

ABOARD A MILITARY AIRCRAFT — The U.S.-led coalition launched one of the largest airstrikes yet on an Islamic State car bomb factory Thursday night, hitting a sprawling compound south of Mosul with aircraft dropping dozens of bombs.

The airstrike was the culmination of months of intelligence gathering and is part of a broad effort to weaken militants in Mosul in advance of a U.S.-backed offensive to recover Iraq’s second-largest city from the Islamic State.

The U.S. Air Force allowed a reporter to fly in a surveillance aircraft over Syrian airspace as the planes attacked the targets in neighboring Iraq.

The Airborne Warning and Control System (AWACS) plane monitors radio traffic and provides radar coverage in crowded airspace above Iraq and Syria, coordinating among coalition planes and issuing warnings if Russian or Syrian aircraft appear heading near allied operations.

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The aircrew was hunched over banks of radar screens for hours at a time as the aircraft circled above Syria, helping to refuel tankers and fighter planes, ensuring the combat aircraft remain at safe distances from each other.

Car bomb factories represent an increasing worry for the coalition because the Islamic State has managed to sneak the bombs into Baghdad and use them to disrupt Iraqi ground offensives. A recent bombing in Baghdad killed more than 200 civilians.

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U.S.-backed Iraqi forces are massing troops south and north of Mosul, though the final assault into the city may be months away.

“We are actively shaping the battle space for Iraqi security forces while degrading Daesh's ability to terrorize innocent civilians,” said Lt. Gen. Jeffrey Harrigian, who commands coalition air operations in the Middle East, referring to the Islamic State using an Arabic acronym. “These are the kinds of (truck bombs) that were used to kill more than 200 people in Baghdad."

Staff officers made plans to strike the factory with 17 aircraft, including U.S. B-52 bombers, F-16s and Navy and Marine Corps F/A-18s and several flights of coalition combat planes.

They planned to hit 43 points within the compound after military planners carefully studied imagery and other intelligence.

The compound once had been a chicken farm before it was taken over by the Islamic State and converted into a factory for improvised explosives that have become a trademark of the war.

Typically, these so-called “deliberate targets,” which are planned in advance, take an average of between 45 to 60 days before they are vetted and approved, according to the Air Force.

The aircraft came from bases throughout the region and a Navy carrier in the Persian Gulf. The planes linked up with tankers to refuel them.

Some of the aircraft experienced mechanical difficulties on the way to the target area and had to return to base, requiring commanders to quickly reassign some of the targets to the other aircraft participating in the airstrike only moments before it was scheduled to commence.

Even after dozens of bombs were dropped, some buildings were still standing. Commanders reassigned a pair of A-10 attack planes who came in and hit the targets still erect.

All 43 targets within the compound were struck, the coalition said.