Air traffic control (ATC) is a service provied by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC systems worldwide is to separate aircraft to prevent collisions, to organize and expedite the flow of traffic, and to provide information and other support for pilots when able. 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. Integration to the United States becan circa 1929 with a man named Archie League. Mr. League controlled aircraft from his home field of St. Louis using colored flags while standing in a wheelbarrow.