In 1956, the RCA's Halakhah Commission, consisting of R. Joseph B. Soloveitchik, R. Samuel Belkin and R. Hayyim Heller, was set to issue an official opinion on whether Orthodox rabbis may sit on the Synagogue Council of America, a body consisting of rabbis from all movements. Five days before the scheduled meeting of the Halakhah Commission, the rabbinic advisory board of the Rabbinical Alliance, consisting of prominent roshei yeshiva, issued a prohibition on participationg in the Synagogue Council. In response to this announcement, the RCA's Halakhah Commission responded to the inquiry presented to it as such (Community, Covenant and Commitment: Selected Letters and Communications, pp. 151-152):
[T]he question submitted to us... was temporarily taken off the agenda of the Halakhah Commission. Our decision not to take action at the present time is motivated by the fact that under the present circumstances that now prevail and for which we bear no responsibility, it is humanly impossible to discuss impartially this most serious matter and to render an opinion meeting high standards of halakhic objectivity and truthfulness.Students and followers of R. Soloveitchik have long been offended by this preemption of his decision and been impressed with his humble silence in not actively fighting his colleagues (see also here).
I was told that the following on this subject by R. Amos Bunim was recently published in Yated Ne'eman and has been republished by the Five Towns Jewish Times (link):
As a talmid and musmach of Rav Soloveitchik, I remember his vociferousness in speaking out against any relationship with the Conservative and Reform Jews on any and all religious matters.In this context, it is worth noting R. Walter Wurzburger's recollections on this (link):
I once heard from Rav Aharon Kotler in 1956, when 11 rashei yeshiva met and issued an issur against the Synagogue Council of America and the New York Board of Rabbis. Rav Kotler was the chairman of that meeting, and he requested that there be no mention of the issur until he had a chance to discuss the matter with Rav Soloveitchik.
However, an overly zealous individual (not one of the rashei yeshiva) found out about the issue and publicized it before Rav Kotler had the opportunity to meet with Rav Soloveitchik. Rav Kotler was very upset and he told me, “This day is a tragic day in the history of Judaism in America.”
After the petirah of Rav Soloveitchik, I went to Brookline, Massachusetts, to be menachem aveil. The family told me the depth of Rav Soloveitchik’s hurt stemming from the fact that he was not included with the other rashei yeshiva.
It has been reported that while the Rav opposed the continued membership of Orthodox groups, the Rabbinical Council refused to abide by his instructions. To point out the absurdity of this claim, one need only take into consideration the indisputable fact that as the chairman of its halakha commission, the Rav was the unchallenged halakhic authority of the Rabbinical Council of America. I cannot help but be amused by fanciful accounts of the Rav's views on the issue. I vividly recall a session with the Rav and the late Rabbi Klavan, when we mapped strategy to prevent the Union from seceding from the Synagogue Council.
The Rav's opposition to moves which threatened the unity of the Jewish community also manifested itself in his attitudes towards non-Orthodox groups. He counselled against denying Conservative or Reform Rabbis the right to use communal mikva'ot for conversions. Moreover, he once instructed me that Reform conversions that were accompanied by circumcision and immersion in a mikve had to be treated as a safek giyur, (Accordingly, a get would be required to dissolve a marriage in which one of the partners previously underwent a Conservative or Reform conversion which conformed to the requirement of mila and tevila).