Mildenstein had taken an early interest in Zionism, even going so far as to attend Zionist conferences to help deepen his understanding of the movement. He actively promoted Zionism as a way out of the official impasse on the Jewish question: as a way of making Germany Judenrein (free of Jews).
Some Zionists, whose movement had grown tremendously in popularity among German Jews since Hitler came to power, co-operated.
On 7 April 1933, the Juedische Rundschau, the bi-weekly paper of the Zionist movement, declared that of all Jewish groups only the Zionist Federation of Germany was capable of approaching the Nazis in good faith as "honest partners".
The Federation then commissioned Kurt Tuchler, an acquaintance of Mildenstein, to make contact with possible Zionist sympathizers within the Nazi Party. Tuchler hoped to convince von Mildenstein's circle that the Nazis should openly promote Jewish nationalism. Tuchler asked von Mildenstein to write something positive about Jewish Palestine in the press. Mildenstein agreed, on condition that he be allowed to visit the country in person, with Tuchler as his guide. So, in the spring of 1933 a party of four set out from Berlin, consisting of von Mildenstein, Tuchler, and their wives.
They spent a month together in Palestine, and von Mildenstein began to write a series of articles for Der Angriff, a Nazi Party newspaper in Berlin, founded by Joseph Goebbels in 1927. According to Lenni Brenner, Von Mildenstein himself remained in Palestine for a total of six months before his return to Germany, and He even learned a few words of Hebrew.
In August 1933 Hitler's government and German Zionists entered into the Haavara Agreement, which encouraged emigration by allowing Jews to transfer property and funds from Germany to Palestine.
On his return to Berlin, von Mildenstein's suggestion that the solution to the Jewish problem lay in mass migration to Palestine was accepted by his superiors within the SS.
Between 9 September and 9 October 1934, Der Angriff published a series of twelve quite pro-Zionist reports by Mildenstein, entitled A Nazi Goes to Palestine, in honor of which the newspaper issued a commemorative medallion, cast with the swastika on one side and the Star of David on the other.
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From August 1934 to June 1936, von Mildenstein worked in the headquarters of the Sicherheitsdienst (SD), the security service of the SS, in Section II/112, in charge of the Jewish Desk, with the title of Judenreferent (Jewish Affairs Officer). This title meant that he was responsible for reporting on "Jewish Affairs" under the overall command of Reinhard Heydrich.
During those years, von Mildenstein favored a policy of encouraging Germany's Jewish population to emigrate to Palestine, and in pursuit of this policy he developed positive contacts with Zionist organizations. SS officials were even instructed to encourage the activities of the Zionists within the Jewish community, who were to be favored over the assimilationists, said to be the real danger to Nazism.
Adolf Eichmann, later one of the most significant organisers of the Holocaust, believed that his big break came in 1934, when he had a meeting with von Mildenstein, a fellow Austrian, in the Wilhelmstrasse and was invited to join Mildenstein's department. Eichmann later stated that Mildenstein rejected the vulgar anti-semitism of Streicher. Soon after his arrival in the section Mildenstein gave Eichmann a book on Judaism by Adolf Böhm, a leading Jew from Vienna.
In the summer of 1935, then holding the rank of SS-Untersturmführer, von Mildenstein attended the 19th Congress of the Zionist Organization in Lucerne, Switzerland, as an observer attached to the German Jewish delegation.
Von Mildenstein's apparently pro-Zionist line was overtaken by events, and after a dispute with Reinhard Heydrich in 1936 he was removed from his post and transferred to the Foreign Ministry's press department. He had fallen out of favor because migration to Palestine was not proceeding at a fast enough rate.