Smythe was in Nanking during the Battle of Nanking and its aftermath, the Nanking Massacre. He served as Secretary of the International Committee for the Nanking Safety Zone from December 14, 1937 to February 10, 1938. In that role, he and the Committee's chairman, John Rabe, recorded the atrocities committed by Japanese troops and made daily reports complaining to the Japanese embassy. Smythe reported that while the Japanese embassy continually promised to do something about the atrocities, it was not until February 1938 that anything substantive was done to restore order to the city.