This article traces the history of an Ottoman legal custom related to the construction of sultanic (imperial) mosques. According to conventional narratives, the victory over non-Muslims was the essential requisite for constructing a sultanic mosque. Only after having emerged victorious should a sultan use the funds resulting from holy war to build his own mosque. This article argues that this custom emerged only after the late sixteenth century in tandem with rising complaints about the Ottoman decline and with the ḳānūn-consciousness of the Ottoman elite, although historical accounts present it as if it existed from the beginning of Ottoman rule. It rapidly gained importance, so much so that the Sultan Ahmed Mosque was dubbed “the unbeliever’s mosque” by contemporary ulema. After having examined details of the custom’s canonization, the article deals with how it left its imprint in construction activities (struggles and workarounds), historical sources, literature, and cultural memory, up to the nineteenth century.