Formed in 1931, SD was administered as an independent SS office between 1933 and 1939, when it was transferred over to the Reich Security Main Office. Reinhard Heydritch was appointed chief of the SD by Heinrich Himmler in August 1931.
When Adolf Hitler came to power in January 1933, Himmler wasting no time set the SD in motion, they began creating a vast card-index of the regime's political opponents, arresting a number of Socialists, Communists, journalists and Jews, sending them to the newly established concentration camp at Dachau.
THE FUNCTION AND GENERAL STRUCTURE OF THE SD
It was tasked with the detection of actual or potential enemies of the Nazi leadership, and to neutralise any such opposition, whether internal or external. The SD developed an army of agents and informants throughout Germany and in the occupied territories. The main function of the SD was as an intelligence operation, it carefully tracked foreign opinion and criticism of Nazi policies, censoring them when possible, with hostile comments and cartoons in the SS weekly magazine Das Schwarze Korps.
In addition tasks assigned to the SD as well as the Gestapo were to keep tabs on the morale of the German population at large, Adolf Hitler in particular was very sensitive regarding what he considered the importance of keeping the German population on his side, as a result regular reports including opinion polls, press dispatches and information bulletins were established The SD was also tasked with looking for dissension, and to deal with it.
The most sinister aspect of the SD was its control of the Einsatzgruppen which played a leading role in the mass shooting of Jews in occupied Eastern Europe. Inland SD [The Interior Security Service] was where Adolf Eichmann began implementing the “Final Solution to the Jewish Question”.
Reinhard Heydritch was the main co-coordinator of the Wannsee Conference in January 1942, where plans were laid out for the "Final solution of the Jewish problem"
Much like the Nazi revolution in general. Membership in the SD appealed more to the impressionable youth, most had served in the military, and were generally well educated, with about 14% earning doctorate degrees, leader Heydrich viewed the SD as the “cream of the cream”.