This U.S. Navy Escort Group Changed the Course of World War II in the Pacific
Admiral Soemu Toyoda needed answers. The newly appointed commander in chief of Japan’s Combined Fleet, Toyoda found himself facing several unpleasant facts. By May 1944, Allied naval and air strength in the Pacific Ocean was growing at an alarming rate. Already, fast-moving enemy forces had advanced far across northern New Guinea and into the Admiralties and through the Marshall Islands in the Central Pacific.
Toyoda could not yet determine whether the next American thrust would head north into the Marianas or continue west toward Palau and the Philippines. The six carriers, 10 battleships, and 40 other warships of his First Mobile Fleet could crush an enemy advance, but those vessels carried only enough fuel for one decisive sea campaign. Before sending Japan’s last remaining surface force into battle, Toyoda required hard evidence of American naval activity and intentions.
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Much had changed since the heady days of 1941 and early 1942. Japanese long-range patrol aircraft, once able to roam far into Allied territory, could now only rarely penetrate the enemy’s air defense umbrella. Radio interception, so useful during the war’s first months, was rendered virtually useless by advanced American communications security procedures. That left submarines as Toyoda’s sole reliable means of reconnaissance.
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Unfortunately, Japan’s largest, most capable fleet subs—the oceangoing I-class boats—were increasingly being pressed into service as transports hauling food and supplies to Imperial Japanese Army garrisons marooned by leapfrogging Allied forces. Scouting duties would have to be performed by the smaller Ro-class submersibles of Rear Admiral Noboru Owada’s Submarine Squadron Seven. These vessels were designed for coastal patrol, however, and lacked the surface radar systems Owada deemed so necessary for conducting reconnaissance missions.
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What their crews did not lack was courage. Each Ro-class boat then anchored at Saipan in the Marianas held between 40 and 60 sailors, the cream of the Imperial Japanese Navy undersea force. Combat veterans all, these well-trained seamen posed a substantial threat to any Allied vessel caught in their periscope sights.
Yet Owada’s orders were to locate and report enemy warships not sink them. He directed his boats to picket a 200-mile track between New Guinea and the Caroline Islands labeled the NA Line. Should they spot an Allied armada steaming toward the Philippines, these scouts were sure to radio back with positive confirmation. Armed with this intelligence, Admiral Toyoda could then order his Combined Fleet into the climactic battle he believed would win victory for Japan.
On May 15, 1944, the seven Ro-class boats of Submarine Squadron Seven departed Saipan to take up stations along the NA Line. Their 650-mile voyage would take six days and was tracked closely both by Owada’s staff on Saipan and Combined Fleet headquarters in Japan.
The progress of Squadron Seven was followed by another group of naval officers, listening from a heavily guarded facility at the American naval base at Pearl Harbor, Hawaii. These men belonged to Fleet Radio Unit-Pacific (FRUPac), the top-secret signal intelligence center responsible for collecting and decoding all enemy radio communications intercepted by the U.S. Navy. Already FRUPac had helped win a stunning American victory at Midway, not to mention its role in Operation Vengeance, the ambush of Admiral Isoroku Yamamoto by U.S. Army Air Forces Lockheed P-38 Lightning fighters in 1943. This brilliant team of mathematicians, puzzle solvers, Japanese linguists, and electronics experts was about to change history once again.
A routine radio transmission, made on May 13, 1944, set in motion what would become one of the most epic battles in the annals of antisubmarine warfare. This short, encrypted message came from Lt. Cmdr. Yoshitaka Takeuchi, captain of the fleet sub I-16. Takeuchi’s report, plucked from the airwaves by American technicians, advised Admiral Owada that his vessel was due to arrive with food and supplies for the bypassed garrison at Buin on the southwest tip of the island of Bougainville on May 20.
FRUPac analysts deciphered enough of Takeuchi’s dispatch to estimate his course and time of arrival at Buin. This information quickly made its way to Admiral William F. “Bull” Halsey’s Third Fleet headquarters, also at Pearl Harbor, for action. Halsey had to move fast, though, since intelligence such as this was extremely perishable. Countless factors from weather to mechanical breakdowns to unpredictable sea conditions might put I-16 miles from where the Americans thought it was. And just because FRUPac knew the whereabouts of an enemy sub did not mean the U.S. Navy could get hunter-killer teams there quickly enough to find and sink it.