He participated in the London Naval Conference 1930 as a rear admiral and the London Naval Conference 1935 as a vice admiral, as the growing military influence on the government at the time deemed that a career military specialist needed to accompany the diplomats to the arms limitations talks. Yamamoto was a strong proponent of naval aviation and served as head of the Aeronautics Department, before accepting a post as commander of the First Carrier Division. Yamamoto opposed the Japanese invasion of northeast China in 1931, the subsequent full-scale land war with China in 1937, and the Tripartite Pact with Nazi Germany and fascist Italy in 1940. As Deputy Navy Minister, he apologized to United States Ambassador Joseph C. Grew for the bombing of the gunboat USS "Panay" in December 1937. These issues made him a target of assassination threats by pro-war militarists.