[2] In 1879, Iwamoto graduated from the Imperial Japanese Army Academy and eventually achieved the rank of first lieutenant (中尉, chūi).

In 1887, he was discharged from the Army on account of his association with members of the Freedom and People's Rights Movement including Inukai Tsuyoshi.

[3] Iwamoto and Ishibashi Usaburō [ja] founded the Siam Colonization Society (暹羅植民協会, Shamu Shokumin Kyōkai) with financial support from Chaophraya Surasakmontri.

[2] Before Iwamoto returned home to recruit Japanese peasants for the colonization project, Surasakmontri gave him a golden sword to be sharpened in Japan.

[2] In 1897, Iwamoto published a book about his travels — A True Account of an Expedition to the Three Countries of Siam, Laos, and Annam (暹羅老檛安南三国探検実記, Shamu-Raosu-An'nan Sangoku Tanken Jikki).