长州藩
Tsuboi Kōzō was born as Hara Kōzō, the second son of a doctor in what is now part of Hōfu, Yamaguchi, and was adopted into the Tsuboi family as a child. As a Chōshū Domain "samurai" he took part in the defense of the city during the bombardment of Shimonoseki by European warships from September 5 to 8, 1864. Witnessing firsthand the firepower and devastation caused by a relatively few Western warships, Tsuboi became convinced Japan must also obtain this weaponry to survive. He enlisted in the Chōshū domain navy later that year and served aboard the "Kigai-maru" while studying the English language and navigation at the Chōshū Naval School.