This paper examines the actions of Italian immigrants in Buenos Aires and Montevideo to support Italy’s mobilisation during the First World War. It focuses on immigrant institutions that participated in activities including military recruitment and welfare collections to assist the Italian side. It also investigates ways Italian immigrants collaborated across the Río de la Plata to mobilise war-related resources. Through its analysis, this article narrows in on a neglected period of time in Italian immigration historiography and uncovers ways events in Italy might have affected immigrant behaviours. It explores the degree of integration that existed between these two communities and within a transnational immigrant network built around ‘Italian’ notions of belonging. More broadly, this paper illustrates the value of scholarly focus on periods of crisis in immigrant homelands. The study of such periods helps advance understandings of social relations within immigrant communities and the transnational networks in which immigrants are situated.