In September 2019 a swarm of 18 bomb-laden drones and seven cruise missiles overwhelmed Saudi Arabia's advanced air defenses to crash into the Abqaiq and Khurais oil fields and their processing facilities. The surprisingly sophisticated attack, which Yemen's Houthi rebels claimed responsibility for, halved the nation's output of crude oil and natural gas and forced an increase in global oil prices. The drones were likely not fully autonomous, however: they did not communicate with one another to pick their own targets, such as specific storage tanks or refinery buildings. Instead each drone appears to have been preprogrammed with precise coordinates to which it navigated over hundreds of kilometers by means of a satellite positioning system.