In 1937, the Hama-born writer and publisher Jubran Massuh sent a letter from the city of Tucumán in Argentina to the Maronite Patriarch Antonius Arida. In addition to two copies of his recent book al-Masīḥī wa al-muslim (The Christian and the Muslim), included as a gift, the three-page letter contained a plea for the Patriarch to prepare to confront Muslim fanaticism (taʿaṣṣub). Only two years after expressing his doubt about Muslim promises of “brotherhood” and “equality” to the Patriarch, however, Massuh seemed ready to leave behind this pessimism. He had, by then, met the exiled Antun Saadeh, founder of the Syrian Social Nationalist Party, and was impressed enough with the party’s anti-sectarian principles to become the editor of al-Zawbaʿa, its periodical in Argentina. At this time, Massuh declared in Buenos Aires’s El Diario Siriolibanés that Khalid Adib, a Syrian Nationalist from Tripoli who accompanied Saadeh, was not at all a fanatic and the first Muslim he could genuinely call his brother. This paper situates Massuh’s apparent about-face in the context of his published writings in the nineteen thirties and forties. It will consider what we can learn about sectarian politics in this era from a diasporic perspective. What informed Massuh’s perceptions of Muslim “fanaticism” from Argentina? What do his ideological shifts – including an interest in socialism after he left the Syrian Nationalists in the mid-forties – reveal about transnational middle-class responses to burgeoning Arab-Islamic nationalist mass politics? The case of this Syrian Argentine thinker suggests that affective encounters with Muslims in diaspora may have played an under-appreciated role in coloring contemporary attitudes about majoritarian politics. To trace how Massuh’s thinking in this regard developed over the course of roughly a decade, the paper will consider some of his published articles and books before, during, and after his affiliation with the Syrian Nationalists in a first attempt at an intellectual biography of this tumultous period in his life. It will also offer a close reading of Massuh’s 1937 letter, excavating the continuities between in his published works and this private sectarian polemic.