The U.S. Army was prepared. They detected the firing location and requested the dispatched of a Predator drone equipped with Hellfire missiles. The Department of Defense releases video of the attack (sub par quality but viewable) and provides the timeline of events:The part about no legal approval is also a big step forward. Stupid worrying about "legal approval" prevented us from nailing Mullah Omar in the early days of the Afghanistan campaign.
5 Sept 0458 60mm mortar fired at Balad Airbase
5 Sept 0459 Point of origin from mortar attack is determined
5 Sept 0500 Joint Tactical Air Controller contacted for UAV support
5 Sept 0502 Runner joins four others near to house, hot mortar tube visible
5 Sept 0508 Hellfire missile fired
Ten Minutes.
Within one minute, the firing location of the mortar tube is determined. Somewhere between two to four minutes, a Predator is on site, and begins shooting video. Within ten minutes the terrorist safe house is identified and attacked. All of this was done without a soldier having to get out of his seat or the need for legal approval.
Here's the low-res video of the robot drone attacking the terrorists who fired the mortar.