In this lecture, Bensaïd focuses on the history of the Fourth International in its founding years, which means 1933 and 1938. First of all, he introduces a historical motivation to understand what battles we come from, and what is the problematic heritage we have. Secondly, he points to present interest in the problems that were tackled by the FI in its founding years. He organises the lecture around three points: when did a new international become necessary; on what basis was the new international built; and lastly, how was it built? He discusses the problems of entryism and fusion with a focus on Trotsky's writings on these issues.