The impulse behind Islamic ecumenism the rapprochement between Sunnis and Shi‘as in modern Iranian history originated in the activities of Sayyid Jamal al-Din al-Afghani (d. 1896), and his endorsement of Sultan ‘Abdu’l-Hamid’s notion of ittihad-i Islam (unity of Muslims). The ecumenism of al-Afghani and his immediate followers was essentially political in motivation, inviting Muslims to form a united front against British encroachment. In contrast, the later Iranian attempts at promoting solidarity between Sunnis and Shi‘a was motivated by primarily religious causes. During the twentieth century, and before the Islamic Revolution of 1979, two Iranian waves of ecumenism can be discerned. The first was fostered by “reformist” theologians such as Sayyid Asad Allah Kharqani (d. 1936 ), and by the efforts of Shari‘at Sangalaji (d. 1944) during the decades following the Constitutional Revolution, particularly in the 1930s. The second originated in the activities of Grand Ayatollah Haji Aqa Husayn Burujirdi (d. 1961), the sole marja‘-i taql?d (Source of Emulation) of the Shi‘a during the 1950s. For Kharqani, recovery of a united Islamic front like that which, in his mind, had existed in the first forty years of the religion’s history, was essential in strengthening Muslim interests against threats posed by multiple “Others.” These latter forces of unbelief (kufr) were “the followers of old religions, new religions, the materialists, and naturalists.” Sangalaji, similarly, projected onto the Baha’i minority the extremism (ghuluw) of which the Sunnis accused the Shi‘ites themselves. His objective was to endow mainstream Twelver Shi‘ism with the status of orthodoxy and thus to effect a rapprochement between it and “the other orthodoxy,” embodied in mainstream Sunni Islam. The origin of Burujerdi’s ecumenical commitment, on the other hand, was far more focused. A vitriolic anti-Bahaism is said to have been the passion of his life. For him, Islamic rapprochement was the logical coming together against a heretical common enemy of both Shi‘ites and Sunnis. Based on an in-depth study of the writings and biographies of the figures involved, this paper argues that the rise and expansion of the Baha’i faith created a perceived threat that induced Iran’s Islamic establishment to see Shi‘a-Sunni rapprochement as a necessary defensive maneuver.