U.S. military sources tell Noah Shachtman of the Danger Room that one of our fighters shot down an Iranian drone over Iraq last month.

Details of the incident remain sketchy, and there was no media reporting on the subject prior to Shachtman’s account. But the shoot down provides yet another reminder of the “shadow war” fought between the United States and Iran, amid the larger conflict in Iraq.

Since American forces entered Iraqi territory six years ago, Tehran has spared no effort to target our personnel. Operatives from Iran’s Qods force have supplied funding and weapons to insurgents, including rockets and explosively formed projectiles (EFPs), used in the deadliest IEDs that destroyed hundreds of military vehicles and killed scores of our troops. In response, U.S. special forces have quietly targeted Iranian operatives, arresting dozens and killing others.

Now, as Mr. Shachtman reports, the shadow conflict has entered another phase, with the recent drone engagement. Iran has long accused U.S. aircraft of illegally entering its airspace, but this is the first time we’ve acknowledged that Tehran’s drones are operating over Iraq, and subject to intercept by our fighters.

The Iranians operate several different types of UAVs, which are used primarily for surveillance. Tehran’s most widely deployed drones include the Ababil (“Swallow”) which has a wing span of 10 feet and a cruising speed of roughly 160 knots. Iran has also produced a smaller drone, the Mirsad, which has been exported to Hizballah units in Lebanon.

Tehran also claims that it has developed a new, stealthy UAV, with a range of up to 600 miles. But Iran often exaggerates its military capabilities, and reports of the stealth drone have not been confirmed.

But there is little reason to doubt the shoot down story. The Iranians want to keep an eye on American forces in Iraq and with the insurgency in serious decline, Tehran has fewer opportunities for “eyes on the ground.” Additionally, platforms like the Ababil can cover more territory–assuming they remain undetected.

By sending UAVs over Iraq, Iran may be trying to exploit perceived weaknesses among U.S. air defenses assets. Patriot batteries left the country years ago, and much of the radar coverage comes from ground-based assets, rather than AWACS. That means our radar “picture” is subject to coverage gaps caused by terrain, ducting and other factors.

Still, we managed to find that Iranian UAV and shoot it down, which is no mean feat. With their small size and slow cruising speeds, drones are notoriously difficult to detect. U.S. SIGINT operators used to watch–and listen–with amusement as Saddam’s air defense units tried to track and engage our Predator UAVs. Even with vectoring from ground controllers, Iraqi fighters typically flew past their targets. Once in a great while, the Iraqis managed to knock down a Predator with AAA fire, but we can’t recall a single, successful air-to-air intercept, despite dozens of attempts.

Successfully engagement of a drone is (typically) a multi-faceted operation, combining signals intelligence, radar tracking, the right air-to-air weapons and a skilled fighter pilot. We’re guessing that SIGINT providing initial cueing and tracking through the intercept/exploitation of radio conversations, and guidance signals unique to the drone. With that information, air or ground-based weapons controllers knew “where to look,” and vector the fighter to the right location.

Despite those advantages, a successful intercept was hardly assured. It’s likely that both the radar platform and the fighter pilot probably had to adjust the Doppler gates on their radar, to compensate for the UAV’s slow speed.

The weapon (most likely) used to down the drone was an IR-guided missile–probably an AIM-9X. Radar tracking issues would make for a difficult AMRAAM shot, and modern fighters carry about 500 rounds of ammunition for their on-board cannon. With burst “limiters,” that’s enough for no more than 3-4 passes, assuming you can maintain track on the UAV.