After working for a Jewish politician, Wiener served as a soldier in World War I. Horrified by the far-right extremism and anti-Semitism that aggravated after Germany’s defeat, he began to write and campaign to warn people about the consequences of growing hatred. From 1919-1933 he worked with the Central-Verein deutscher Staatsbürger jüdischen Glaubens (Central Association of German Citizens of Jewish Faith) to fight anti-Semitism. After reading Hitler’s Mein Kampf in 1925, he concentrated his efforts on the Nazi Party, which he saw as the most dangerous force in German politics. In 1933 Wiener fled with his family to the Netherlands and set up the Jewish Central Information Office (JCIO) in Amsterdam, which collected and disseminated evidence of the Nazi assault on Jewry and the fate of Jewish refugees.