The Hedgehog: The Royal Navy's Secret Weapon to Kill Japanese Submarines

Warfare History Network

Security, Asia

Tokyo hated this during World War II.

In the Spring of 1944, Japanese Admiral Soemu Toyoda assembled a large fleet of warships at Tawi-Tawi in the southern Philippine Islands. There was no doubt in his mind that Allied military forces would continue their westward drive across the Pacific, but he was uncertain as to the direction of the next attack.

General Douglas MacArthur, commander of the Allied forces in the Southwest Pacific, had firmly established beachheads in New Guinea, and Japanese scout planes had reported that an American task force was gathering in the neighborhood of the Marshall Islands. Toyoda realized that the U.S. naval force assembling in the Marshalls could strike at Guam or Saipan in the Marianas—or MacArthur, using New Guinea as a base of operations, could attack the Palau Islands.

American Assault on the Palau Islands

By this stage of the war, the Japanese Navy would have had a difficult task defending both sectors at the same time. Therefore, Toyoda decided to choose Tawi-Tawi—because of its central location—for his fleet buildup. From there, the Japanese admiral would be able to send his forces in either direction.

In May 1944, the Japanese High Command received Intelligence that Manus Island, in the Admiralty group, was being geared up as a springboard for an American assault on the Palau Islands. MacArthur’s troops were also reported gathering at points along the New Guinea coast. Even so, Toyoda felt that an attack on the Marianas remained a distinct possibility. He needed to learn, with exact certainty, in which direction to send his fleet, and an operational plan was quickly implemented.

Toyoda established a submarine scouting line extending from Truk Island in the Carolies to a point just west of Manus. His submarines were stationed at designated intervals along the line and positioned so that any invasion fleet could, hopefully, be detected. The vessels assigned to that operation were I-16, RO-104, RO-105, RO-106, RO-108, RO-116, and the RO-117.

The Hedgehog: Forward-Throwing Anti-Submarine Mortars

Toyoda reasoned that he had all bases covered. The odds appeared even except for two factors—the arrival of a squadron of new U.S. Navy destroyer escorts (DEs) and the infamous Hedgehog weapons with which they were armed.