by Attack on Pearl Harbor: Strategy, Combat, Myths, Deceptions
(5) US Technical Mission to Japan Ordnance Data Page for 250-kg GP bomb37
The GP bomb could be effective against aircraft carriers and cruisers. The bombs were fitted with a fuze with a 0.2 second delay, sensitive enough to initiate upon hitting the relatively thin decks of a cruiser or carrier. The bomb would penetrate 20 to 40 feet to detonate deep in the hull. In contrast, American 500- and 1,000-pound GP bombs had a 1/10th second delay, expecting 3 to 8 feet of penetration before exploding. These weapons were designed more to tear up a carrier’s flight deck and shut down flight operations.
For comparison purposes, American estimates in the secret document “Striking Power of Air-borne Weapons” give an insight into the power of similar weapons. Three hits with 500-pound GP bombs had a 99% chance of sinking a 1,630-ton destroyer, and a 70% chance of sinking a 2,100-ton destroyer. Three hits by 1,000-pound GP bombs had a 95% chance of sinking an Atlanta class light cruiser, and a 30% chance of sinking a 10,000-ton heavy cruiser. Six hits by 1,000-pound GP bombs would give an 80% chance of sinking a heavy cruiser.38
Why did Japan not develop an armor piercing bomb that could be employed by a dive bomber? The US Navy in 1941 had four such bombs from 1,000-pound down to 600-pound. A 1,000-pound bomb was introduced in October 1942 that could penetrate a 5-inch deck if dropped from 6,500 feet in a 300-knot 60-degree dive.39 The exact reason is unknown, though there are some facts that may have influenced the decision (if it was contemplated at all):
1. Tests in early 1941 indicated that an 800kg bomb had to be dropped from 12,000 feet to get the penetration needed to defeat a battleship’s deck armor. It may have been thought that a 250kg bomb, the design payload of the D3A Val, dropped from lower altitude in a dive, just would not be able to penetrate enough armor.
2. A shortage of special steel precluded stocking specially-made 800kg AP bombs. A steel shortage might also have deterred work on lighter AP bombs for dive bombers.
SEAD: GP Bombs and Strafing v. Air Defenses
Prior to the outbreak of the war in the Pacific, there was considerable uncertainty regarding the effectiveness of air attack against well-defended warships. Defensive fire was expected (by surface officers) to disrupt air attacks and exact horrible attrition from bombers. Some Japanese believed that the interferences from defensive fires would be so unsettling that bomber hit rates would be one-third that achieved in peacetime exercises, and losses to defensive fires would be unacceptably high.40
Torpedo bombers were thought to be particularly vulnerable, as they were large targets traveling slowly at low altitude and directly toward their targets, offering a greatly simplified AA fire control problem.
One solution was thought to be the employment of fighters and dive bombers to suppress AA fire. Fighters would strafe the superstructures of warships, killing or driving away AA gun crews, damaging the weapons themselves, and interrupting their ammunition supply. GP bombs exploding in a ship’s upper works could be devastatingly effective against exposed gun mounts and their personnel. A 250kg bomb hit amidships on a battleship would stop most of the fire from the 5-inch and 3-inch anti-aircraft guns that were generally mounted on an open deck and unshielded. One of the highest priority modifications after the Pearl Harbor attack was to install splinter shields and gun tubs around AA guns and their crews.
Suppression of enemy air defenses (SEAD) attacks against warships were a recognized tactic during the pre-war period. The US Naval War College Maneuver Rules issued in March of 1940 included provisions for strafing to suppress warships’ AA fire. An attack by 12 fighters (each with two machine guns and two 100-pound bombs) would reduce a ship’s AA fire by 30% for a full day; six or more strafing aircraft would reduce the effectiveness of AA batteries by 50% against any bombing attack that immediately succeeded the strafing.41 In June 1944 this rule was revised, reducing the required number of strafing aircraft to three, likely a reflection of the heavier .50-cal six-gun armament on modern fighters.42
The Japanese employed SEAD during their war against China, sending fighters against Chinese AA emplacements. This employment perhaps reflected more the lack of enemy fighters, leaving Japanese fighter pilots looking for something to do. It may not have been planned. They were not particularly effective in that role, since their training and mindset concentrated on air-to-air combat.
By 1939 the naval air forces incorporated suppression of enemy AA defenses into a system of massed aerial assault against fleet units. Fighters would strafe the ships’ AA defenses, followed by level bombers, and then the near-simultaneous attack of dive bombers and torpedo bombers. The dive bombers, less vulnerable than the torpedo bombers, were assigned to both sweep away AA batteries with their 250kg bombs, and to draw AA fire away from the torpedo bombers.43 These tactics were available to the Pearl Harbor planners.
One thing that undermined the effectiveness of the 250kg bomb in the SEAD role was its fuze setting. The 0.2 second delay meant that the bomb was supposed to penetrate well below decks before exploding, which would not be as effective in suppressing AA as the Americans’ super-quick fuzing, which swept a ship’s upper works with bomb fragments even if the bomb exploded in the water close aboard.
The fuze delay led some American sailors to suppose the Japanese GP bombs were delayed-action AP bombs. For example, the destroyer Dale was leaving the harbor during the dive-bombers’ attacks on Nevada. Three of Dale’s sailors recalled:
There were bombs falling all around. And they were armor-piercing bombs, which buried themselves deep in the mud on the bottom of the channel before blowing up. The explosions sent huge fountains of water and stinking mud up higher than Dale’s radio mast…
The bombs that they were using were 16-inch armor-piercing battleship rounds with fins welded to them…
One hit to starboard, and the other fell into the water right next to the boat davit where I was standing. The explosions sent up a huge fountain of stinking mud that fell all over us.44
The last AP bombs were dropped 45 minutes before Dale sortied.
Testimony of Dale’s sailors highlights one of the problems in using eye-witness reports from veterans’ memories. They can be contaminated by later reading and by their discussions amongst themselves. Sailors did not know during the attack that the Japanese were using converted battleship projectiles as AP bombs, and they certainly did not know if the fins were welded or threaded or heat-shrunk on. They remembered huge columns of mud, and they recorded they were attacked by AP bombs, since in US practice AP bombs had delayed fuzes and GP bombs did not. In this way witness reports have introduced inaccuracies into the historical record.
The Type 99 GP bomb sometimes detonated with a red cloud-like signature. Observers claimed that some appeared to be filled with flammable liquid,45 while several officers reported them as an incendiaries.46 Shaw took three hits described as “liquid-filled incendiaries weighing from 200 to 300 lbs.”47 The bomb that hit forward of the Pennsylvania also had noteworthy incendiary effects. However, the endorsement to Shaw’s War Damage Report correctly concluded, “There is no positive evidence that incendiaries were used elsewhere in the attacks on ships at Pearl Harbor, and it is believed that none were dropped on Shaw.”
A more likely answer is that the explosive pour in the bomb was done haphazardly, leaving bubbles and discontinuities, so that the explosive detonated incompletely, some burning like fireworks. Although spectacular in appearance, such displays were less lethal than if the explosive had properly detonated.
Air to Air
The A6M Zero comprised a quantum change in fighter performance. It was armed with two 7.7mm (.30-cal) machine guns firing through the propeller, and two 20mm cannon, one in each wing. The kinetic energy and explosive charge of the 20mm made it effective in the ground attack role. Ammunition was loaded with alternating armor piercing and tracer rounds, thought to be particularly effective in penetrating aircraft fuel tanks and then igniting the leaking fuel. The fuze was very sensitive, as it had to explode immediate
ly after impact with light aircraft surfaces.
The 20mm carried only 60 rounds per gun, about 7 seconds of fire. The 7.7mm (.30-cal) guns carried 680 rounds per gun,48 for about 40 seconds of fire. Pilots were trained to fire the 7.7mm guns first, and when on target add the 20mm for the killing stroke. In practice this could rarely be done successfully, because the initial velocities of the guns were mismatched (600 m/s 20mm rounds chasing 750 m/s 7.7mm rounds), and the velocity decay rate of the rounds were drastically different.
These defects were noted. The A6M3 Type 0 Model 32 Hamp (an updated model of the Zero) introduced an upgraded 20mm cannon with a longer barrel giving more initial velocity and better matching the 7.7mm’s ballistics. The aircraft designers provided 100 rounds per gun. The A6M3 Hamp was introduced in April 1942, four months after Pearl Harbor.
The primary opposition to the A6M Zeros at Pearl Harbor was the P-40B Warhawk. The P-40B was armed with two .50-cal machine guns firing through the propeller and two .30-cal machine guns in each wing, a total of six machine guns. It was not as maneuverable as the A6M Zero, but had armor protection for the pilot, protected fuel tanks, and could take a tremendous amount of punishment. Used properly, the P-40B was an adequate-to-good fighter. If they could overwhelm or avoid the defending A6M Zeros and penetrate to the Japanese bombers, the lightly-built, vulnerable bombers would have suffered significant losses and significantly reduced weapons delivery accuracy.
A B5N Kate carrier attack bomber armed with a torpedo taking off from a carrier. This photograph has sometimes been identified as depicting one of the bombers from the Pearl Harbor attack. This is more likely a single frame from a propaganda movie about the attack. The torpedo looks an exercise round: note the prominent dent in the warhead. Source: Naval Archives, Washington DC
CHAPTER THREE
WARGAMES
First Estimates
Early in April 1940 Yamamoto’s staff officer for air discussed the attack with his opposite number on the Second Carrier Division’s staff, Lieutenant Commander Suzuki Eijiro. Suzuki believed that, with surprise, the attack could succeed. However, he believed that it would cost the Japanese three of the four carriers used in the attack, perhaps all four. Suzuki calculated that if the Americans sent up 100 bombers, that Kido Butai’s defenses could eliminate only about 40, leaving 60 to bomb the fragile carriers.1
Wargames
The first test of the attack’s operational concept was in a series of wargames conducted in September of 1941. While other staff members in the main facility gamed the Southern Advance, 30 selected personnel—a group of the most senior commanders—withdrew to a separate room.
The specific rules used in the wargame have not surfaced, but from some of the surviving commentary it is possible to understand the basic structure the Japanese used.
First, some background. There were three styles of wargames prevalent among the navies of the period:
• Rule-based games, where all movement and combat was regulated and adjudicated strictly in accordance with specific, predetermined rules. Combat results were determined by deterministic rules, or alternately a throw of the dice or random number tables provided variability;
• Umpire-controlled games, where combat results were decided by the judgment of subject matter experts or specifically-trained officers;
• Hybrid games, a combination of rule-based and umpire-controlled games, where rule-based or randomly generated combat results can be overruled or modified by the umpire.
The leading practitioner of naval wargaming was the United States Navy. The Naval War College (NWC) held academic and analytic wargames using rules first introduced in the early 1920s and constantly updated. Wargames at the tactical and operational level constituted the majority of the time spent in instruction in the senior officer’s course, lasting nine months to a year. By 1941 the rules and data filled hundreds of legal-sized pages. In contrast, the British naval wargaming rules ran to 43 pages.2 The USN also had a simplified system used during fleet exercises.
Rules were strictly followed. An umpire had the authority to adjust results in the course of the game for those cases not handled thoroughly in the rules (which were few). When testing operational plans, the atmosphere was very interactive, with all ranks providing input and criticism. Calculations of logistic requirements were an integral part of the games.
At the NWC, strategic map wargames and tactical wargames were used for training, to test out and explore options in war planning, and to compare the capabilities of potential ship designs, test out tactics, and formulate fighting doctrine. Operationally, commanders used games to test plans for weaknesses, practice responses to enemy countermoves, and explore logistics concerns. For American naval officers, the games were intense—in the schoolhouse, reputations were made and broken; underway, admirals who blundered in fleet exercises were known to be passed over for promotion or gracefully retired from active service.
The Japanese ran hybrid games, primarily to test operational plans to see how they stood up to enemy countermoves.
The Japanese wargames could also be an intensely political process. In the case of the games testing the Pearl Harbor plans, Yamamoto used gaming as an opportunity to socialize the plan with his senior commanders and the Naval General Staff. The game introduced the plan’s concept, showed its potential, and corrected conceptual and operational errors. It allowed Yamamoto to muster his supporters and, more significantly, to flush out the opposition, drawing them into making their positions public. This prevented situations where officers could work against the plan behind the scenes—the game forced them to make their positions open and public, so Yamamoto could identify them and deal with their objections.
The plan could be changed on the spot to address some officers’ issues. In other cases, with the opposition identified, Yamamoto could employ his considerable charm, and appeal to them directly for their support. Several key opponents identified during the games were convinced to withdraw their opposition.
This socialization process was of considerable significance, considering the sometimes volatile nature of Japanese decision making. For example,
When Rear Admiral Yamaguchi Tamon, the commander of the Second Carrier Division [Hiryu and Soryu], became aware that planners were considering not including his command in the strike force bound for Pearl Harbor, he became highly intoxicated and physically seized Nagumo and, watched by a collection of officers who made no attempt to intervene, beat his commander until the latter agreed that Yamaguchi’s formation would be included in the task force. In another confrontation, Yamaguchi threatened to kill Nagumo.3
Surprisingly (by Western standards) Yamaguchi retained his command. His desires were accommodated, not as a result of his conduct but because his conduct aligned with Yamamoto’s desires. His behavior could be interpreted as the kind of “fighting spirit” the Japanese valued.
The wargaming environment also conferred a certain amount of control to the host, in this case, Yamamoto. A few words with the umpires might influence results to favor those options Yamamoto desired, and show undesired options in a bad light. With control of the assumptions and combat results, a good umpire could govern the outcome, both of the game and of the political process.
Japanese wargames tended to push logistic matters to the background. Logisticians could attend and observe the games to see what support would be required of them, but clearly they were subordinate to the combat commanders. Logistics did not drive the scenarios, as in the NWC games. The Japanese Pearl Harbor games did not totally neglect logistics, but did assume that the logistics problems would be overcome.
The Japanese staff system valued accord and promoted unanimity. Agreement with the group was a guiding principle. Dissenters had strong social and professional pressures to concur with the group. As a result, after agreement it was very difficult to later challenge assumptions or judgments. Anyone attempting such impudence would at best firmly be put in his place, at worst publicly reprimanded.
Commanders might be required to execute a plan that they felt was seriously flawed.
Prange’s Misinterpretation
Japanese wargaming during World War II has been given a bad reputation by Prange, who criticized the wargames that preceded Pearl Harbor and especially those held before the Battle of Midway.
In the workup to the Midway operation, Prange highlighted a game where the Japanese force was attacked and two carriers were lost. Rear Admiral Ugaki Matome, the Chief of Staff of the Combined Fleet, intervened and restored the two carriers.4 In Prange’s words, “With a sunny lack of realism… He did not scruple to override unfavorable rulings of other umpires.”5 Prange contends such actions were the result of overconfidence and a refusal to credit the Americans with a reasonable level of combat effectiveness. Concentrating on personalities rather than process, Prange characterizes the wargame as a contest of one-upmanship. He scolds what he sees as “war games rigged to make the enemy look incompetent, just as in the planning for Pearl Harbor.”6
Prange did not understand operational wargaming. His interpretation is seriously flawed.
One purpose of an operational wargame is to understand what could possibly go wrong and to gather insights on how a plan might progress in the face of enemy countermeasures. Umpires’ decisions are revised to take into account modifications of the plan, lessons learned, to prevent the game from departing too far from what is considered the most probable course of events, or to allow different elements of the plan to be exercised.