Allied submarines in the Pacific War

Allied submarines were used extensively during the Pacific War and were a key contributor to the defeat of the Empire of Japan.

During the war, submarines of the United States Navy were responsible for 55% of Japan's merchant marine losses; other Allied navies added to the toll. The war against shipping was the single most decisive factor in the collapse of the Japanese economy. Allied submarines also sank a large number of IJA troop transports, killing many thousands of Japanese soldiers and hampering the deployment of IJA reinforcements during the battles on the Pacific islands.
They also conducted reconnaissance patrols, landed special forces and guerrilla troops and performed search and rescue tasks. The majority of the submarines involved were from the U.S. Navy, with the British Royal Navy committing the second largest amount of boats and the Royal Netherlands Navy contributing smaller numbers of boats.

The Allied submarine campaign is one of the least-publicized feats in military history, due in large part to the efforts of Allied governments to ensure their own submarines' actions were not reported in the media. The U.S. Navy adopted an official policy of unrestricted submarine warfare, and it appears the policy was executed without the knowledge or prior consent of the government. The London Naval Treaty, to which the U.S. was signatory, required submarines to abide by prize rules (commonly known as "cruiser rules"). It did not prohibit arming merchantmen, but arming them, or having them report contact with submarines (or raiders), made them de facto naval auxiliaries and removed the protection of the cruiser rules. This made restrictions on submarines effectively moot.

...
Wikipedia