During World War I, in which Portugal joined the Allies, Caldas had one of three internment camps in the country. In 1916, most Germans in Portugal were deported, but men aged 15–45 were imprisoned to prevent their joining the German military. Originally, all of the approximately 700 prisoners were shipped to Angra do Heroísmo, on Terceira Island in the Azores, where they were held at the Fortress of São João Baptista. In 1918, to reduce overcrowding at the fortress, 168 internees were moved to Caldas, where they stayed in military barracks located at the Parque D. Carlos I (Pavilhões do Parque). The prisoners were released the following year, after the end of the war.