The Berlin Mosque was the first permanent place of Muslim worship in Germany. Never a purely local affair, the construction of the Berlin Mosque depended on the legacies of imperialism and the shifting geopolitical contexts of the 1920s. International diplomats and former Wilhelmine and Ottoman agents living and working in Weimar Berlin made sense of the mosque project through categories and ideas forged in the decades before the First World War. They gradually recalibrated their ambitions when confronted, as they were, with radical transformations of the Muslim world, from the dissolution of the Ottoman Empire and the abolition of the caliphate to the emergence of new political leaders from Arabia to Afghanistan. This article demonstrates how the construction and uses of the Berlin Mosque closely followed how diplomats and other actors, both German and non-German, assessed the geopolitical potential of a German–Muslim alliance in the post-Ottoman, post-Wilhelmine moment.