Born in 1928 to fight antisemitism, the National Conference for Christians and Jews aimed to be a key-player in post-war peace. The US organization expanded in 1946 into the International Council of Christians and Jews and used the new American diplomacy in Europe to foster its own influence. Through its conferences in 1947 and 1948, ICCJ promoted Jewish-Chr. brotherhood as part of a larger plan: religious freedom and interfaith education would foster democracy abroad and hence reinforce international peace. Aiming to develop amicable relations among religious leaders and others on international platforms (UNESCO), this faith-based lobbying had yet little success beyond the Seelisberg moment. Examining the political and religious factors behind the ICCJ's failure, this paper sheds light on early Cold War ecumenism. It outlines inherent tensions between Americans and Europeans, mainline Protestants, Jews and Catholics, NGOs and the Vatican, when it came to interfaith relations. ICCJ's financial support entailed stiff monitoring of local Jewish-Chr. fellowships in continental Europe. Grassroots movements in France, Germany, Italy, Switzerland soon called out the US-based policy, leading to the complete withdrawal of US funding and strong competition between ex-members. ICCJ's understanding of religious freedom framed by NCCJ's mainly Protestant consistency missed its point in Europe: too religious for the UNESCO and the New Education Fellowship, it led to indifferentism according to Catholic bishops, while some local sections aimed to extend dialogue with Communists. The promotion of faith-based brotherhood out of anticommunism didn't last in front of doctrinal concerns from Rome. Yet, as the Holy Office condemned ICCJ in 1950, it had already split and disappeared.This crisis had long-lasting consequences on interfaith relations, up to VaticanII. Focusing on power relationships entailed in funding, we analyse anew the dynamics that shaped such "brotherhood diplomacy"