This chapter examines how the Institut Rousseau and the International Bureau of Education (IBE) became key actors of educational internationalism between 1918 and 1932, linking New Education and peace education. The study is based on the AIJJR Archives and IBE Archives (correspondence, minutes, conference records). It shows how Édouard Claparède, Pierre Bovet, Adolphe Ferrière, Alice Descoeudres, Jean Piaget, Pedro Rosselló and Marie Butts built transnational networks, founded the International School of Geneva, and structured the IBE to coordinate global educational cooperation. The chapter uses social history of internationalism, history of educational knowledge, and transnational circulation approaches. It highlights tensions between universalist pacifist ideals and national-cultural rootedness among these reformers.