Subsequently, together with the Italian Socialist delegates (Oddino Morgari, Giacinto Menotti Serrati, and Angelica Balabanoff among them), Rakovsky was instrumental in convening the anti-war international socialist Zimmerwald Conference in September 1915. During the congress, he came into open conflict with Lenin, after the latter voiced the Zimmerwald Left's opposition to the resolution (at one point, Rakovsky reportedly lost his temper and grabbed Lenin, causing him to temporarily leave the hall in protest). Later, he continued to mediate between Lenin and the Second International, a situation from which emerged a circular letter that complemented the "Zimmerwald Manifesto" while being more radical in tone. In October 1915, he reportedly did not protest Bulgaria's entry into the war—this information was contradicted by Trotsky, who also indicated that the "Tesniatsy" had been the target of a government crackdown at that exact moment.