During World War I, he worked to smooth relations between Japan and the other nations allied against Germany, Austria-Hungary and Turkey. He worked, when ambassador to France, for Japanese allegiance to the allied declaration swearing not to negotiate a separate peace with the Central Powers. As special ambassador to the United States during the war, he worked to ease the increasing tensions over Japanese actions in China and American actions toward Japanese living in the United States. He reflected the view, widely held among Japan’s liberal internationalists, that Japan and the United States should accommodate each other’s interests in potential areas of friction. In 1917, with Robert Lansing, he negotiated the agreement that bears their names. Although both men believed their countries had more to gain from cooperation than rivalry, neither was able to operate as a free agent since their respective governments did not believe true compromise possible at the time. Thus, the Lansing-Ishii Agreement had no real substance. As ambassador to the United States, he continued to grapple with Japanese-American tensions concerning the Siberian Intervention and the differences that arose at the Paris Peace Conference. He died in 1945.