From Code Girls, by Liza Mundy (Hachette, 2017), Kindle pp. 249, 250-251:
November 1943, one month after Dot’s arrival at Arlington Hall, marked the war’s most devastating month for Japanese tonnage sunk. U.S. subs sank forty-three ships and damaged twenty-two. American sub captains received intelligence of seventy-six movements of enemy ships. In December, American subs sank or damaged about 350,000 tons, including thirty-two ships sunk and sixteen damaged.
Behind the success of the U.S. Navy were the code breakers. “The success of undersea warfare is to a certain extent due to the success with which Japanese code messages were translated,” noted a naval report. An American naval commander pointed out in a postwar memo that sometimes a convoy might slip through, but only because U.S. submarines were kept so busy by information from decoded messages that they could not handle all the convoys they were alerted to. Over at the Naval Annex, the assembly line of WAVES identified the movements of marus [merchant marine ships] supplying the Japanese Navy. Findings from both operations found their way to the submarine captains, who could hardly keep up with the bounty of intelligence.
The devastation of Japan’s shipping had an enormous impact. Soldiers were deprived of food and medicine. Aircraft did not get spare parts and could not launch missions. Troops did not reach the places they were sent as reinforcements. On March 12, 1944, a broken 2468 message gave the route and schedule of the Twenty-First Wewak Transport convoy, sunk while leaving Wewak to return to Palau. When the Japanese Eighteenth Area Army made a “complete tabulation of shipping from Rabaul and Truk during January,” in an attempt to convince Japanese Army headquarters that it was feasible to send them much-needed supplies, these messages laid out the shipping routes and sealed their doom. Only 50 percent of ships reached the destination; only 30 percent got home.
At the end of the war, a U.S. naval report found that “more than two-thirds of the entire Japanese merchant marine and numerous warships, including some of every category, were sunk. These sinkings resulted, by mid-1944, in isolation of Japan from her overseas sources of raw materials and petroleum, with far reaching effects on the capability of her war industry to produce and her armed forces to operate. Her outlying bases were weakened by lack of reinforcements and supplies and fell victim to our air, surface and amphibious assaults; heavy bombers moved into the captured bases.” This report’s author, C. A. Lockwood, commander of the submarine force of the U.S. Pacific Fleet, noted that his men got a “continuous flow of information on Japanese naval and merchant shipping, convoy routing and composition, damage sustained from submarine attacks, anti-submarine measures employed or to be employed, effectiveness of our torpedoes, and a wealth of other pertinent intelligence.” Whenever code breaking was unavailable, he added, “its absence was keenly felt. The curve of enemy contacts and of consequent sinkings almost exactly paralleled the curve of volume of Communication Intelligence available.”
He added: “There were many periods when every single U.S. sub in the Pacific was busy” responding.
In fact, he added, code-breaking intelligence made it seem to the Japanese that there were more American submarines in the Pacific than there really were. “In early 1945 it was learned from a Japanese prisoner of war that it was [a] common saying in Singapore that you could walk from that port to Japan on American periscopes. This feeling among the Japanese was undoubtedly created, not by the great number of submarines on patrol, but rather by the fact, thanks to communications intelligence, that submarines were always at the same place as Japanese ships.”