In 1916, following Romania's entry into World War I, Rainer returned to Bucharest and rejoined Marta, being called up for medical service as a colonel. During the 1916 counteroffensive, the Rainers set up surgery wards in the hospitals that had become full to overflowing with wounded from the Battle of Turtucaia and the zeppelin attack on Bucharest. Later, during Bucharest's occupation by the Central Powers, Francisc Rainer established temporary hospitals in schools. He continued teaching under the German administration, so that his students would not miss a year, but was also obliged to teach separate courses for German students. He managed to prevent the occupying authorities from requisitioning the faculty's possessions. Viewed as political suspects, the Rainers were singled out for internment in Bulgaria, but spared by Minovici's intervention on his behalf. Marta Rainer, who managed war hospitals, organized teams of surgeons to help deal with the humanitarian crisis, and operated non-stop during the extremely cold winter of 1917–1918.