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During World War I, Hayashi served as Minister Plenipotentiary to China. In 1919–1920 Hayashi served as the first civilian governor of the Kwantung Leased Territory. In 1920, he was reassigned to London, and in 1921 was part of the Japanese delegation to the League of Nations Assembly in Geneva, Switzerland. Hayashi served as Ambassador to the United Kingdom in 1920–1925, and in that capacity represented his country at the Genoa Conference in 1922.