What role did US submarines play against Japan in WWI?

 (My answer on Quora)

A very significant one. In fact, according to historian Euan Graham, 55% of Japan’s merchant marine losses were courtesy of US submarine attacks. The Imperial Japanese Army (IJA) troop carriers were also attacked with regularity further hindering the Japanese war effort. The net effect of these attacks was to expedite the collapse of the Japanese economy and hinder Tokyo’s military progress.

However submarine warfare didn’t start out that well for the US initially. The 1941–42 Philippines campaign was somewhat of a disaster with US submarines failing to notch a single strike against their Japanese targets. There were operational problems associated with the Mark 14 torpedoes. Fortunately and thanks to the efforts of vice-admiral Charles Lockwood these were corrected.

One of the reasons for US submarine success during the war was that the Japanese failed to utilize the convoy system as well as the Allies did in the Northern Atlantic. Since Japan was extremely reliant on seaborne supplies the impact was significant.