Juventudes Socialistas Unificadas (JSU) - Unified Socialist Youth was a youth organization formed in the spring of 1936 in Spain through the amalgamation of the Spanish Socialist Workers' Party (PSOE) and Communist Party of Spain (PCE) youth groups. Its leader, Santiago Carrillo, came from the Socialist youth, but had secretly joined the Communist youth prior to the merger, and the group was soon dominated by the PCE.
References
Unified Socialist Youth Wikipedia(Text) CC BY-SA