After serving in the army during the first world war, Pflugk-Harttung had become a member of the Freikorps, the paramilitary organizations that sprang up around Germany as soldiers returned in defeat from World War I. The Freikorps were the key Weimar paramilitary groups active during that post war time. Many German veterans felt disconnected from civilian life, and joined a Freikorps in search of stability within a military structure. Kapitänleutnant Horst von Pflugk-Harttung along with his brother Heinz were two such volunteers. The German volunteer movement was opposed to the communist Spartacists movement. During this period Pflugk-Harttung became friends with the future head of the Abwehr, Wilhelm Canaris. In 1919, Pflugk-Harttung along with his brother was accused of being directly involved in the murders of Spartacists Rosa Luxemburg and Karl Liebknecht. Both men were acquitted but evidently many thought them guilty, for the brother was assassinated himself sometime later.
Sweden
In 1931 Pflugk-Harttung had been recorded as helping to coordinate fascist groups and organisation in Sweden. The Swedish authorities had Pflugk-Harttung expelled after it was discovered that he had been importing armaments illegally into Sweden for Munckska kåren. Pflugk-Harttung then went to Norway on a similar mission where again he was soon asked to leave.
Denmark
By 1933 Pflugk-Harttung was working for German Intelligence in Denmark. As a cover he worked as correspondent for the paper, Berliner Boersen Zeitung, which was an organ of the Reich War Ministry. Amongst his many covert tasks, he kept a close eye on the German exiles in Denmark, whilst the Danish Police co-operated with him through go-betweens. Along with other postgraduates of Gestapo spy schools, Pflugk-Harttung set up a spy ring that operated secret broadcasting stations and had engaged in nautical and hydrographical research. Between them they had drawn up maps and charts, graphs and complicated mathematical tables of data that required the best technicians even to understand. They communicated by complex code systems that changed frequently. The outlay for so extensive an apparatus as theirs could be justified only as part of Third Reich preparation for War against major countries. Pflugk-Harttung's network watched and reported on British shipping movements into and out of the Baltic Sea. In 1938 information revealed by Ernst Wollweber to the Danish authorities, along with further investigations by the Police led to the arrest of Pflugk-Harttung, along with eight other Germans and three Danes who were charged with operating as spies in Copenhagen. Investigations proved that the spy ring had been involved in the sabotage and sinking of Spanish trawlers on behalf of General Francisco Franco and his Nationalist navy which was operating from German ports. These actions included the utilization of the spying apparatus to shell and sink the SS Cantabria off the Norfolk coast by the Nationalist Auxiliary cruiser Nadir. For his part in the espionage Pflugk-Harttung was only sentenced to a year and a half in prison and was released after a few months owing to German government pressure. After his release from prison, Pflugk-Harttung became one of the leading German Intelligence chiefs in Denmark.
World War II
In 1944, Pflugk-Harttung was in control of the Marinesonderdienst in Bordeaux. He was arrested by U.S. forces and taken to Arizona as a prisoner of war. Interrogators concluded that Pflugk-Harttung was "a patriotic German, but traditionally and violently opposed to all things Socialistic, Communistic and, above all, Asiatic. He is not a Nazi." He was held until November 1947, after which he returned to Germany.
