Part 1/2

The Imperial Japanese Navy (IJN) rather infamously severely underinvested in commerce protection and anti-submarine warfare, but also had rather strong institutional blinders on that shaped both their perception of and their response to the submarine threat.

Perhaps the first and most important issue to discuss is the IJNs own institutional focus on battle above all else. The IJN conceived of its primary mission as winning the great decisive battle against the Americans, and thus dedicated itself almost entirely to preparing for that engagement. While this resulted in the excellent IJN carrier arm and night battle tactics, it also pulled focus away from any aspect of naval warfare that was not directly related to fighting that great surface engagement. Logistics suffered immensely, as did commerce protection, as these area of warfare tended to receive far less attention from the IJN in terms of resource--both human and otherwise--than did the IJN's fleet units. The IJN In the words of Evans & Peattie in Kaigun:

…the Japanese Navy neither understood nor prepared for war at all. Rather…it prepared for battle.

The impact of this, of course, was that the IJN concerned itself primarily with destroying warships, as sinking or damaging enemy warships would contribute to winning the decisive battle in a way that sinking merchantmen would not. For the IJNs submarines, this manifested in both their design and mission objectives. I go into more detail on the IJN's conceptions of its submarine force and their intended mission here, but suffice it to say, the IJN intended its submarines to be engaged with shadowing and attacking the American battlefleet as it moved westward across the Pacific, rather than engaged in warfare against Allied merchant shipping. As such, the wolfpack tactics of the Germans, designed to inflict damage on Allied merchantmen transiting the Atlantic was of little interest to the Japanese. As I mention in the linked post, the IJN was not completely unaware of the importance of anti-submarine warfare. In fact, during World War I, a unit of Japanese destroyers had gone to the Mediterranean to aid the Allies in escorting convoys there. This Japanese force proved very effective at its mission, to the extent that the Royal Navy requested that more Japanese crewed ships be deployed for convoy escort duty. Yet, in the aftermath of that war, IJN leadership war far more interested in the lessons of Jutland and what it meant for their intended large surface engagement against the Americans than in the lessons of anti-submarine warfare in the Mediterranean. The Japanese Naval War College expressed some interest in studying the problems of shipping protection, but these attracted relatively little attention. Focus for academic study of shipping protection fell to the Navy Torpedo School (more concerned with the offensive application of torpedoes) and eventually to the Navy Mine School, which was a far less prestigious and under supported institution. Training was similar, and while the Japanese Navy trained extensively, practically none of this was in anti-submarine warfare or escort duties. This institutional bias meant that in the IJNs preparations for the war, it spent almost no time or effort considering what it would have to do to counteract a sustained campaign against Japanese commerce.

Of course, the Japanese were aware that submarines could pose a threat to Japanese merchant shipping, even if the Navy wasn't particularly interested in investigating the exact extent to which they would be. It took till mid-1941, due to prodding by the Cabinet Planning Board (a government office responsible for industrial mobilisation) seeking numbers on which it could plan for the IJN to conduct an actual assessment of anticipated merchant shipping losses. The IJN's analysis was slapdash, simply multiplying the rates of U-boat sinking in World War I by the IJN's own estimate of American sub density to come to an anticipated loss of 2.7 million tons of merchant shipping over three years of war. While these figures were incredibly optimistic, with little grounding, in the early stages of the war, it seemed that the IJNs estimation was accurate. In 1942, American submarines sank 884,928 tons of Japanese merchant shipping, slightly above the anticipated losses, but hardly a catastrophic rate of loss. Yet, these relatively low rates of losses had more to do with Allied limitations than any kind of Japanese effort at countering the submarine threat. American submarines were equipped with the infamous Mark 14 torpedo, which suffered from a wide variety of faults that significantly reduced the effectiveness of American submarines. In addition, what forward bases the submarines could operate from (such as Fremantle and Midway) were initially ill equipped to support submarine operations and were far from the vital Japanese shipping lanes. The combination of these factors helped to conceal the underlying flaws of the Japanese merchant protection system and the vulnerability of the Japanese merchant marine.