Pacific War:

U.S. submarines, as well as some British and Dutch vessels, operating from bases at Cavite in the Philippines (1941–42); Fremantle and Brisbane, Australia; Pearl Harbor; Trincomalee, Ceylon; Midway; and later Guam, played a major role in defeating Japan, even though submarines made up a small proportion of the Allied navies—less than two percent in the case of the US Navy. Submarines strangled Japan by sinking its merchant fleet, intercepting many troop transports, and cutting off nearly all the oil imports essential to weapons production and military operations. By early 1945 Japanese oil supplies were so limited that its fleet was virtually stranded.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". What did submarines sinking Japanese ships do?
strangled Japan