On April 18, during an unannounced visit to Baghdad, U.S. Secretary of Defense Ashton Carter announced an additional 217 American troops and a fleet of AH-64 Apache helicopters will be deployed to Iraq to assist local forces in the upcoming assault on Mosul, which is widely expected to be one of the bloodiest — if not the bloodiest — battles of the campaign against the Islamic State thus far.

“The fight of Iraq is the fight for Mosul,” an anonymous senior U.S. defense official told Reuters shortly before Carter’s Baghdad visit on April 18. “It’s a very large urban scenario … We are going to need to be more aggressive, the Iraqis are asking us to be more aggressive.”

That willingness to be more aggressive is perhaps most evident in Carter’s decision to order Apaches helicopters to participate in the battle for Mosul. This would be the first time Apaches have been used in sustained combat against ISIS, as the Iraqi government rejected offers to deploy them in Ramadi. According to The Associated Press, there will be eight Apaches “authorized to help the Iraqi forces when Iraq leaders determine they need them.”

Armed with a 30mm chain gun, 70mm rockets, and Hellfire missiles, the AH-64 Apache is lethally accurate at up to 12 kilometers. The addition of eight Apaches would give the Iraqis a significant advantage as it attempts to penetrate the heavily fortified ISIS stronghold.

However, Apaches have been shot down by enemy fire in both Iraq and Afghanistan on numerous occasions over the years, and there’s a legitimate risk of that happening in Mosul as well.

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Mosul is the second largest city in Iraq and the Islamic State’s de facto headquarters outside of Syria. It fell to the terrorist group in June 2014 after government forces abandoned their posts en masse, leaving behind three divisions’ worth of American arms and military equipment, including about 2,300 armored Humvees, which, naturally, ISIS absorbed into its vast arsenal.

On the day Mosul fell, local military, police, and security officials told Reutersthat the ISIS militants who overran the city were armed with anti-aircraft weaponry. Several months later, in October 2014, ISIS released photos purporting to show its fighters using a Chinese-made missile launcher to blast an Iraqi army attack helicopter out of the sky. Defense officials in Baghdad later confirmedthat an Mi-35M had been shot down near the town of Baiji, and that the two pilots on board were killed. It was the second army helicopter shot down that week.

Around that time, ISIS published an online guide describing how to shoot down Apache helicopters, which the United States had just begun deploying to Baghdad. The guide, which has since been removed from the internet, included a diagram of an Apache and instructions on the use of Man-Portable Air Defense Systems, or MANPADS.

According to an article published in The New York Times in October 2014, both U.S. and Iraqi military officials at the time were concerned enough about the threat of shoulder-fired heat-seeking missiles to restrict the use of low-flying aircraft on the battlefield. The authors of the article, which was titled “Missiles of ISIS May Pose Peril for Aircrews in Iraq,” wrote:

Syrian rebels have amassed multiple [MANPADS] since 2012, and the Islamic State has generally had little trouble acquiring any weapon used by Syrian rebels either through purchase or capture, military analysts say. Though the Pentagon’s Central Command acknowledges this concern, it said it had no conclusive evidence yet that the Islamic State had such weapons.

If the Islamic State does, in fact, possess surface-to-air missiles, the weapons will certainly have made their way to Mosul, where U.S. officials estimate there are about5,000 militants dug in for a long and bloody fight. What happens if that turns out to be the case? A downed Apache in a sprawling urban war zone like Mosul could draw a considerable number of U.S. ground troops into the fight — something neither Washington nor Baghdad was willing to risk not long ago.

An official traveling with Carter told CNN that the United States is going to “accept more risk in the coming days in Iraq.”