After the war, Wagener was involved in the planning of an attack against the city of Posen (now Poznań, in Poland), but had to flee to the Baltic countries to avoid arrest. There he merged all Freikorps associations into the German Legion, and assumed leadership after its leader, Paul Siewert, was murdered. After returning to Germany, he was active in Freikorps operations in Upper Silesia, Saxony, and the Ruhr area.
In 1920 he studied economics and managed to broaden his knowledge by traveling abroad. In 1929 Wagener joined the Nazi Party and the Sturmabteilung, having been recruited by his old Friecorps comrade Franz Pfeffer von Salomon. As Kershaw explains, Wagener was able to put his business acumen and contacts to good usage for the Nazis, in this case for the SA:
Wagener had used his business contacts to persuade a cigarette firm to produce "Sturm" cigarettes for SA men -- a "sponsorship" deal benefiting both the firm and SA coffers. Stormtroopers were strongly encouraged to smoke only these cigarettes. A cut from the profit went to the SA ....
He functioned as SA Chief of Staff from October 1929 through December 1930, assuming effective command of the SA for a few months in the wake of the Stennes Revolt until the assumption of command by Ernst Rohm as the new Chief of Staff in early January 1931. In 1933 became a member of the Reichstag.
In January 1931, Wagener led the Political-economic Department of the Nazi Party, and in September 1932 he was appointed the Führer's personal economic advisor. Hitler appointed him Reich Commissar for the Economy from April to June 1933.
By late 1930 or early 1931 Wagener had made a mark on Nazi economic policy. As Patch notes (p. 201-02):
Wagener formulated an original set of economic policies based on Social Darwinist principles in confidential talks with Hitler and succeeded in recruiting many middle echelon industrial managers and owners of small factories for the NSDAP....[A confidential draft by Wagener] embraced the ideal of the "company union" (Werksgemeinschaft) and described the employer as the "Fuhrer" within his factory. All disputes over wages and working conditions would be settled within the "family" of the individual company in the National Socialist state of the future. Trade unions would be responsible merely for vocational training.
Wagener was replaced in his role as Commissioner for Economic Questions by Wilhelm Keppler, as Wagener had become embroiled in "coordination" disputes with leaders of industry after the Nazi assumption of power in January 1933, even forcibly occupying the industry-run trade association Reich Association of German Industry with the intention of shutting it down.
Internal conflicts led to legal proceedings against Wagener in 1933 and 1934 in a case brought before the USCHLA (Party tribunal). After the Night of the Long Knives, Wagener was detained for a short time. Nevertheless, he was rehabilitated, and he went back into the army.
In the Second World War, Wagener served at the front, rising to the rank of major general and division commander. After the war, Wagener found himself first in British and later, from 1947 to 1952, Italian prisoner of war camps. He died in Chieming.
In 1946, while being held by the British, Wagener wrote his memoirs about Hitler and the Nazi Party's early history, entitled Hitler aus nächster Nähe. Aufzeichnungen eines Vertrauten 1929-1932 (known in English as Hitler: Memoirs of a Confidante). His work was not published until six years after his death, in 1977. His memoirs are used, to some degree, by Third Reich historians.
Reference Works 
- Kershaw, Ian (1999). Hitler 1889–1936: Hubris. New York: W. W. Norton & Company. ISBN 0-393-04671-0 ("''Kershaw''").
- Lambert, Angela (2007). The Lost Life of Eva Braun. Macmillan. ISBN 031236654 ("''Lambert''") Check
- Patch, William L. (1985). The Christian Trade Unions in the Weimar Republic, 1918-1933: The Failure of Corporate Pluralism (illustrated edition). Yale University Press. ISBN 0-300-03328-1 ("''Patch''").
- Kershaw p. 348.
- After the first Stennes Revolt in August 1930, Hitler assumed Supreme Command of the SA, having cashiered Wagener's friend and war comrade Pfeffer von Salomon as SA head. Wagener was the de facto leader of the SA from this time until Rohm's return in January 1931. See Stennes Revolt.
- See Lambert p. 237.
- Evans p. 384.