The war of Spain (1936/39) was for the armies of the whole world a laboratory of the modern war to come. The Germans, in particular, tried out the tactical use of the modern bomber. The bomber is able to strike and terrorize anywhere: civil soldiers, but also industries and are not with the shelter behind the line of face. For much then, the bomber is the ultimate weapon able to put at knees any power. It becomes imperative to counter this threat.
The systems known already well, fighters and DCA are improved from this point of view. But it is the technology of radiodetection or R.A.D.A.R. who is proposed. The fact of being able to locate a fleet of enemy planes before it does not achieve its goal will make it possible to counter the effective attack of way.

The principle of radiodetection is the fruit of research undertaken throughout the world since the end of the XIXème century. But it is only into 1934 that the first operational systems are developed and installed on steamers in order to detect the icebergs and ships which cross their road.
The British soldiers are interested especially then in the radars of day before to protect their island from the air incursions, but equip also some with their ships to supervise the sea. Since 1937, the Royal Air Force sets up stations of monitoring, Chain Home, which will end up covering all the British territory in 1940. These radars are rudimentary because they are satisfied to announce formations which approach in their zone of detection but they will be very well used during the battle of England.

Of their dimensioned, the Germans as well focus their research in the day before, ç' is with being said in the passive monitoring of the airspace (Freya system) and of the sea (Mammut system) as in the guidance of the guns of DCA and the projectors (Wurzburg system) and the marine artillery shootings (See-riese).
From an offensive point of view the German radars are generally mobile but the failure of Blitz, and the British air raids in occupied Europe oblige Luftwaffe to set up a network of detection similar to Chain Home English. These radars form part of a vast network of fortifications and monitoring covering the coasts of Norway in the south of France known under the name of «Fortress Europe» according to words' of Nazie propaganda and collaborationnist. The word of «fortress» should not delude us, it is built very slowly and will not be finished in 1944.

A photo catch in 1941 by a reconnaissance aircraft of the RAF announces the presence of unknown radars in Normandy. An operation commando is organized to capture elements of the radars and to thus make it possible to the British scientists to study them...