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The High Cost of Faulty Intel
(Page 2)
Naval History, February 2005

How Intelligence Influenced the Campaign

According to the JICPOA, 13,500 Japanese were on Iwo Jima. Even though aerial reconnaissance indicated a massive build-up through the fall and into the early winter of 1944, analysts claimed that, "[T]here [were] no indications that the garrison [had] been reinforced."6 In reality, about 23,000 Japanese occupied the island.7 This gross underestimation skewed the entire intelligence report concerning enemy capabilities, because analysts based other estimates on the number of enemy soldiers they assumed occupied the objective.
Marine Corps map of Iwo Jima drawn following U.S. occupation indicates the extent of the Japanese defenses.

By scrutinizing photos, analysts discovered only 105 major weapon sites, including antiaircraft (AA) and dual-purpose guns, and 119 hardened weapon positions, including pillboxes and covered artillery. But they knew the Japanese would defend Iwo from more than just 224 weapons positions. After all, it was one of the last outer defenses of the home islands.

Accordingly, since observed intelligence showed only part of the Japanese defenses, analysts relied on reported intelligence to fill the gaps.8 Observed intelligence included things analysts could see on a targeted island, and reported intelligence consisted of things analysts hypothesized were on a targeted island. In the case of Iwo Jima, the captured order-of-battle documents from Saipan led analysts to hypothesize that certain Japanese units occupied the objective. In turn, this led them to consult Japanese unit wire diagrams for information on how many men were in each unit, and how many and what types of weapons they had.

After considering reported defenses, analysts believed that in January 1945, the Japanese on Iwo possessed the following weapons: up to 39 artillery pieces of 75-mm or larger, 24 70-mm howitzers, 18 mortars ranging from 81- to 240-mm, 10 80-mm naval guns, up to 54 AA guns with another possible 33 "other AA guns," 42 to 54 37- or 47-mm anti-tank guns, six rocket launcher positions, and 40 tanks.9 Presumably, these made up the bulk of Japanese artillery and support weaponry.

By February, Marine intelligence had increased many of these estimates.10 The number of artillery guns had risen 100%. The number of coast defense and dual-purpose guns had risen by 100% and 162%, respectively. Japanese AA guns had risen by at least 41%. On the other hand, anti-tank (AT) weapons had decreased by 83%, and machine guns had decreased by 17%. Analysts said that there had been an 87% decrease in open artillery positions and assumed that this was because they were moved into concrete and steel fortifications, which had also increased.11

Even while these estimates indicated significant increases, most of them fell short of the actual situation. In reality, the Japanese had 361 artillery pieces of 75-mm or larger, 65 mortars of 81- to 240-mm, 33 80-mm naval guns, more than 200 20- to 25-mm AA guns, 69 37- or 47-mm AT guns, 70 rocket launchers, and 22 tanks. They also possessed 12 massive 320-mm mortars and 94 AA guns of 75-mm or larger.12


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Marines also met more defensive structures than expected; the JICPOA's pre-action report hardly mentioned them. All over the island, the Japanese had built blockhouses and pillboxes for machine guns and artillery. The report asserted that analysts had observed only 39 pillboxes, 13 covered artillery positions with 4 under construction, and about 170 "rifle pits."13 The same report largely neglected blockhouses, but the Marines' February intelligence report indicated 35 blockhouses on Iwo with 4 under construction. It also said 332 pillboxes were on the island.14 This was an astounding increase of fortifications in the one-month time span between reports.

When the battle ended and JICPOA and fleet intelligence units began their analyses of the island's concrete and steel defenses, there were so many they did not have time to plot them all on a map as they had in past campaigns. Analysts were reduced to discussing numbers in broad terms. For example, the JICPOA's 10 June 1945 Iwo Jima after-action intelligence report devoted a whole section of its analysis just to blockhouses and pillboxes, thereby indicating their abundance and successful employment by the enemy. The same report stated that, while many pillboxes commanded a scant 30° field of fire, "there was a sufficient number of mutually supporting pillboxes to offset the restricted field of fire of each weapon."15

What is more, Marines encountered numerous concrete fortifications in a defensive zone they called the Meat Grinder. Located on the east side of the island in the 4th Division's area of operations, it consisted of three mutually supportive strongpoints: Hill 382, another hill called Turkey Knob, and a natural depression in the ground called the Amphitheater. The Meat Grinder included hundreds of defensive structures, and its Amphitheater, according to the JICPOA after-action report, "contained two terraces and three tiers" of concrete fortifications.16 Overall, the JICPOA's after-action bulletin made 21 references to these defenses in just five pages and summarized them in a separate, five-page, ten-photo pictorial.

This also happened with tunnels and caves, many of which were natural, but most of which seemed to be manmade. Pre-battle intelligence claimed the Japanese would use caves to shelter men and logistics from bombardment. The JICPOA figured that the vast majority of caves on Iwo were located on the northern end of the island. In contrast, the after-action report stated "thousands of caves [were] used for defensive positions." 17 Indeed, most caves on the island housed guns and were nearly impervious to conventional infantry assault. Caves were so numerous on Iwo Jima that intelligence analysts said "that it was impossible to plot them all on a 1:10,000 map."18 In the end, it summarized tunnels and caves in a four-page, 11-photo pictorial.

An armada of U.S. ships, including battleships firing 14- and 12-inch guns, bombarded the landing beaches at Iwo Jima for several days, trying to soften the enemy defensive positions before the Marines landed.

Terrain served the enemy in a capacity similar to caves, and analysts barely mentioned it in pre-battle intelligence. It provided the Japanese with excellent natural fortifications and slowed U.S. infantry movement. Loose and deep sand between the western landing beaches and the opposite shore slowed tank and infantry traffic that made easy targets for Japanese gunners and riflemen.19

Similarly, the northern end of Iwo Jima did not contain many man-made defensive structures, because its terrain was just as formidable as the rest of island. Armed to the teeth, Japanese troops hid in every hole, depression, and crevice. Where the terrain allowed, several enemy soldiers clustered together in natural pillboxes and bunkers armed with machine guns. In other instances, the Japanese embedded tank turrets into the island's volcanic rock to serve as improvised blockhouses. Iwo Jima's terrain also allowed for forward and reverse slope defenses on the same ridgelines.


6. JICPOA Information Bulletin 9-45, p. 2. [back to article]

7. George W. Garand and Truman R. Strobridge, Western Pacific Operations: History of U.S. Marine Corps Operations in World War II, vol. 4 (Washington, DC: Historical Division, Headquarters Marine Corps, 1971) p. 458. [back to article]

8. JICPOA Information Bulletin 9-45, p. 1. [back to article]

9. JICPOA Information Bulletin 9-45, pp. 1-2, “Important Errata Note,” p. 11, and map titled, “Military Installations and Estimated Troop Dispositions of Iwo Jima (Sulphur Island).” [back to article]

10. It is unclear how involved fleet intelligence was in the Iwo intelligence operation, or if the same rotation policy that affected the JICPOA’s photo interpreters affected those of the fleet. Regardless, it is evident fleet intelligence was not able to pick up the slack where the JICPOA proved inadequate. [back to article]

11. Headquarters, Expeditionary Troops, Task Force Fifty-Six, State of Enemy Defenses, Iwo Jima, 13 February 1945, pp. 1-2. [back to article]

12. Garand and Strobridge, Western Pacific Operations, p. 454. This chart does not include every weapon the Japanese had. They also possessed three 20-mm mortars. [back to article]

13. JICPOA Information Bulletin 9-45, 1, plus “Nearshore, Beach, and Soil Conditions, Iwo Jima” map that includes minute detail on Japanese defense installations. [back to article]

14. Headquarters, Expeditionary Troops, Task Force Fifty-Six, State of Enemy Defenses, Iwo Jima, 13 February 1945, p. 2. [back to article]

15. JICPOA Information Bulletin 136-45, Defense Installations on Iwo Jima (Pearl Harbor, HI: 10 June 1945,) p. 5. [back to article]

16. JICPOA Information Bulletin 136-45, p. 4. The majority of Japanese concrete fortifications on Iwo were steel reinforced. [back to article]

17. JICPOA Information Bulletin 136-45, p. 19. [back to article]

18. JICPOA Information Bulletin 136-45, p. 2. [back to article]

19. The tactical battle data came from Bartley, Iwo Jima: Amphibious Epic, Garand and Strobridge, Western Pacific Operations, and the chapter on Iwo Jima in Jeter A. Isely and Philip A. Crowl, The U.S. Marines and Amphibious War (Princeton, NJ: Princeton University Press, 1951). [back to article]

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