An MQ-9 Reaper waits to roll out on an ISR mission from Ali Al Salem AB, Kuwait, in July. The remotely piloted aircraft have been a mainstay of US intelligence gathering and strike missions for more than a decade during a time when US air superiority has rarely been tested. Sophisticated air defense and missile systems, however, pose an increasing threat to these aircraft. Houthi rebels in Yemen shot down a Reaper in June and claimed a second downing in August.