Air traffic control (ATC) is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The main job of ATC is to "separate" aircraft to prevent crashes and to make sure aircraft are running on time and as fast as possible. In some countries, ATC may also play a security or defense role (as in the United States), or actually be run entirely by the military (as in Brazil). Air traffic control was first introduced at London's Croydon Airport in 1921. Archie League, who controlled aircraft using colored flags at what is today Lambert-St. Louis International Airport, is often considered the first air traffic controller.