That year in July, Stemmler moved to Spain with her husband, Ernst Goldstein. She began working as a nurse in the International Brigades of the Spanish Civil War at the medical center set up in Murcia. Goldstein engaged in the combat units of the brigades and was killed during the conflict. She worked near the front at field hospitals in Barcelona, Magoria, and Murcia until the war ended in March 1939. Making her way back to France, after crossing the border, she was interned with other German refugees from Spain in the Gurs internment camp near Pau. In 1941, Stemmler was handed over to the Gestapo and transported to Ravensbrück concentration camp, where as a prisoner, she served as a nurse and saved the lives of two Czech prisoners. She remained until her transfer in 1943 to the camp at Auschwitz. Without regard for her own safety, Stemmler used her medical training to treat patients who were suffering from illness or had been subjected to medical experimentation by the Nazis. She was evacuated in 1945 as part of a death march from the camp and was eventually liberated in April by the Red Army.