The German Radio Intelligence Operation were signals intelligence operations that were undertaken by German Axis forces in Europe during World War II. In keeping with German signals practice since 1942, the term "communication intelligence" () had been used when intercept units were assigned to observe both enemy "radio and wire" communication. When the observation of only enemy "radio" communication was undertaken, the term was "radio intelligence" (). The term "intercept service" () was also used up until 1942.
Towards the end of the war, about 12,000 troopers of the German Army were engaged in German Intercept Station Operations during World War II against increasingly powerful enemies. With the decline of information gained by intelligence through aerial observation, prisoner of war interrogations, and reports from enemy agents, communication intelligence became increasingly important. As a result of communication intelligence, German commanders were better informed about the enemy and his intentions than in any previous war.
While this form of electronic warfare was being waged, another aspect also gained steadily in importance, on the high-frequency battlefield of Radar in World War II. Radar used Microwave transmission for the location and recognition of enemy units in the air and on the sea, and each side adopted defensive measures, especially in air and submarine warfare.
The third aspect of this electronic warfare, was the radio broadcasters' war, in which propaganda experts endeavored to influence by providing propaganda against the enemy, through foreign language broadcasts over increasingly powerful transmitters. These three aspects of the radio war continued as a "cold war of the airwaves", even when the guns were silent.
Radio intelligence operations
In addition to the intelligence gained from interception of routine radio traffic in peacetime and the activity intercepted during maneuvers, the political and military events which preceded World War II offered abundant material for interception. The reason for this was the increased traffic between nation-states and the larger number of messages, transmitted by communications equipment in an evolving field. During this period, the German signals intelligence organizations and the specialists employed therein gathered an enormous amount of information. The activities of the Reichswehr intercept service from 1933 to 1939 centered around the various international crises that arose.
Intercept operations during interwar period
Austrian Revolt of 1934
In 1934, at the time of the Austrian revolt, Munich was given the task of monitoring all Austrian communications traffic including internal communications. In its earliest stage, the monitoring and capture of intercepts were intended only to thoroughly acquaint the Reichswehr cipher bureau personnel with Austrian traffic. However, after the revolt began, warrants for the arrest of political offenders appeared in the customs and police traffic, thus giving the Nazi Party information concerning the fate of its partisans across the border. At the same time, the intercept service monitored Italian communications. Its discovery that the Italians were massing a multitude of divisions at the Brenner Pass brought about the timely withdrawal of Germany from the affair.
Occupation of the Rhineland 1935
Ever since 1930, the Munich and Stuttgart intercept stations had, during the summer months at the time of the occupation of the Rhineland, reconnoitered the Demilitarized Zone for suitable intercept sites for use against France. After sites had been selected, the construction work was completed with the help of trustworthy civilians. Several weeks before the Remilitarization of the Rhineland, intensive monitoring of France was ordered. As a result of the intelligence, German troops marched in without fear of reprisal.
The Abyssinian War 1933–1936
From 1933 to 1935, during the period of the Abyssinian War, intercept stations of the Army High Command, monitored Italian traffic without cessation. During the Abyssinian War, the intercept stations and the Wehrmacht cipher bureau were able to follow all Italian traffic very closely because, with one exception, it was in possession of all Italian codes. The strength, organization, and locations of the Italian peacetime army were accurately known, and it was possible to keep abreast of the movements of each unit. The SIS also possessed a roster of all Italian officers, from Commander-in-Chief, Benito Mussolini, down to the youngest second lieutenant. Locations, strength, and armament of the Italian Colonial Troops were also known in detail.