But the outbreak of World War I in Europe saw him summoned to the frontlines that August as a chaplain in which he was later decorated for his actions. He was appointed as a chaplain for the soldiers at the end of September and served in that role until May 1916 when he was permitted a period of rest; he resumed his activities in October. Folci and several others were later taken as prisoners in November 1917 following the defeat of Caporetto and then interned in concentration camps across German regions until 1919 once the Red Cross arrived to oversee prisoner release following the war's end. It was during his time in prison that he tended to his fellow prisoners. He was first held in detention in Rastatt in Baden-Württemberg and received permission to celebrate Mass for himself and his fellow prisoners.