Radar in World War II greatly influenced many important aspects of the conflict. This revolutionary new technology of radio-based detection and tracking was used by both the Allies and Axis powers in World War II, which had evolved independently in a number of nations during the mid 1930s. At the outbreak of war in September 1939, both Great Britain and Germany had functioning radar systems. In Great Britain, it was called RDF, Range and Direction Finding, while in Germany the name Funkmeß (radio-measuring) was used, with apparatuses called Funkmessgerät (radio measuring device). By the time of the Battle of Britain in mid-1940, the Royal Air Force (RAF) had fully integrated RDF as part of the national air defence. In the United States, the technology was demonstrated during December 1934, although it was only when war became likely that the U.S. recognized the potential of the new technology, and began development of ship- and land-based systems. The first of these were fielded by the U.S. Navy in early 1940, and a year later by the U.S. Army. The acronym RADAR (for Radio Detection And Ranging) was coined by the U.S. Navy in 1940, and the term "radar" became widely used. While the benefits of operating in the microwave portion of the radio spectrum were known, transmitters for generating microwave signals of sufficient power were unavailable; thus, all early radar systems operated at lower frequencies (e.g., HF or VHF). In February 1940, Great Britain developed the resonant-cavity magnetron, capable of producing microwave power in the kilowatt range, opening the path to second-generation radar systems. After the Fall of France, it was realised in Great Britain that the manufacturing capabilities of the United States were vital to success in the war; thus, although America was not yet a belligerent, Prime Minister Winston Churchill directed that the technological secrets of Great Britain be shared in exchange for the needed capabilities. In the summer of 1940, the Tizard Mission visited the United States. The cavity magnetron was demonstrated to Americans at RCA, Bell Labs, etc. It was 100 times more powerful than anything they had seen. Bell Labs was able to duplicate the performance, and the Radiation Laboratory at MIT was established to develop microwave radars. The magnetron was later described by American military scientists as "the most valuable cargo ever brought to our shores". In addition to Great Britain, Germany, and the United States,