Submarines of the Imperial Japanese Navy
Imperial Japanese Navy submarines originated with the purchase of five Holland type submarines from the United States in 1904. Japanese submarine forces progressively built up strength and expertise, becoming by the beginning of World War II one of the world's most varied and powerful submarine fleets.
Attack on Sydney Harbour
The Imperial Japanese Navy originally intended to use six submarines in the attack on Sydney Harbour: B1-type submarines , , , and , and C1-type submarines and . The six submarines made up the Eastern Attack Group of the 8th Submarine Squadron, under the command of Captain Hankyu Sasaki.