Complex sentence: He remained at Harvard as an English instructor and assistant dean in charge of freshmen until joining the World War I war effort by going to Washington, DC and opening a Red Cross bureau to assist in reuniting families and locating missing men overseas.
Split into simpler sentences: He remained at Harvard as an English instructor and assistant dean in charge of freshmen. During World War I he opened an American Red Cross bureau in Washington, DC to assist in reuniting families and locating missing men overseas.