During the Pacific War, it was U.S. submarine fleets that gave heavy damages to Japan, which relied on ocean-borne supplies for natural resources. Various submarines were developed and commisiioned at that period, and, among them, the Gato class boats were the main strength. It was in 1940 when U.S. placed an order for 73 Gatos in response to the realization that the U.S. would probably become involved in the war. The first Gato class submarine, Drum, was commissioned on 1 November, 1941 and many of the submarines in its class including the Flasher, Barb, Growler, Silversides, Trigger, Wahoo and Harder became famous durint the war. The Gatos were enlarged version of the preceding Tamber class. They were 350 tons heavier and 1.22m longer, most of the additional weight being taken up by improved diesels and batteries giving improved performance and endurance. having simple structure for mass production, total number of production including its improved variants, Balao class and Tench class, exceeded 200. Most of them carried the burden of the submarine warfare in the Pacific Ocean until the end of the war.