Chapter IV. Intelligence

From: Commander Amphibious Forces, United States Pacific Fleet (Commander Joint Expeditionary Force)

Intelligence planning and preparation of material for the operation against Iwo Jima presented no unusual problems and followed in general the procedure developed during the preceding three operations participated in by this force. Photographic coverage was unusually good, both in quality and quantity. Early receipt of photographs permitted ample time for the preparation of Intelligence material based on photography. An unusually large amount of this material was prepared and issued. In addition to the frequent photographic coverage of Iwo Jima, an excellent submarine reconnaissance was made by the U.S.S. Spearfish. The periscope photography, a sound recording of the actual observations by the sub commander, and soundings taken proved of great value and usefulness.

Previously captured documents and charts provided early and fairly accurate information on terrain, surrounding sea areas, and enemy troop strength. In spite of the unusual amount of advance material it was impossible to accurately determine the exact strength and magnitude of the defensive installations. This was due to the nature of the terrain, effective camouflage, and the fact that the greater part of the defensive installations had been prepared a considerable time before the invasion and had much natural cover. Many of the pillboxes, underground shelters, and entrances to caves had old vegetation growing on and around them.

Air target folders were prepared by ComPhibGrp One and ComAirPac for use by aviators on air support missions, air spotters and observers, and fire support units. These were contact photos with grid and target area shown. On the back of each photo was printed the map of the same area covered by the next photo. These were all bound in a folder, 8 by 8 inches with a cover showing the outline of the various sheets. An aviator looking at the gridded photo, could look at the map (1:10,000) of the same area printed on the back of the sheet he has turned up. These target folders were excellent and most useful. On an island of this size it was possible to do this, but where large areas are to be covered, it will only be practicable to cover the landing area. These folders were distributed separately on a special distribution.

By means of requests to CominCh via CinCPOA, the Atlantic Amphibious Training Command constructed 60 rubber relief models, scale 1:10,000, for use by ships and units of this force. In addition, 40 were constructed for the troops. These models were accurate, and of great value especially to the fire support ships. They were based on the latest and best information available. Ground forms were shown as depicted on the 1:20,000 topographical map.

The organization of four Mobile Hydrographic Units was authorized by the Chief of Naval Operations on 2 December 1943. These units were conceived and organized for the purpose of making rapid advance surveys during assault operations and to perform other functions of immediate importance to the assault units. If considered necessary a final detailed survey is made by a standard naval AGS type of survey ships after capture of the objective.

During the assault on Iwo Jima only two of the four survey vessels were assigned survey duties. The other two continued on screening and patrol work. Hydrographic conditions at Iwo Jima were found to conform closely with those shown on H.O. Chart 6101 and no comprehensive survey program was undertaken. The survey work was confined to searches for reported shoals in the southeastern anchorage area; to the establishment of navigational lights ashore; and some sounding in the mooring buoy area and off PURPLE and BROWN TWO western landing beaches.

It is felt that the Mobile Survey Units are of great value in amphibious operations and that their value will be increased as more involved operation areas are encountered.

The amount of captured material examined was limited and did not measure up to the comparative

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amounts from previous operations. This is contributed to several reasons:

  1. Result of strict orders that no material or documents were to fall in our hands.

  2. The number of caves and pillboxes closed by gunfire.

  3. The slow progress of the advance across the island allowing sufficient time for the destruction of documents of value.

However, all material and documents that were captured were immediately screened for those of immediate value. Those documents of immediate naval interest were sent directly to this command or CTF 53 for translation and use. Others of possible interest were forwarded on to JICPOA.

The planning and execution of public relations arrangements for this operation were on a greatly increased scale over previous operations. A public relations officer was appointed to the Staff for the first time, functioning as a part of the Intelligence Division. CinCPac assigned public relations officers to the Estes, the Auburn and the Hamlin, and temporarily sent a public relations officer to Saipan to facilitate the assignment and reporting of a number of correspondents who joined the force at that port.

The number of correspondents directly assigned to ships of the amphibious force totaled approximately 30, including 4 radio correspondents. The 3 American press associations and Reuters were represented aboard the flagship as were 2 radio correspondents (CBS) representing the combined networks. Many of the leading newspapers and magazines of the country sent individual representatives.

From: Commander Task Group 52.2 (Commander Escort Carrier Force Pacific)

The daily summaries promulgated each day by the Commander Joint Expeditionary Forces (CTF 51) were avidly read and had a decided effect on morale. They are read by all officers engaged in planning or briefing and are paraphrased for all hands. Radio Tokyo's flamboyant and ridiculous broadcasts when compared with the true picture of the progress of battle as described by these summaries gives everyone a good laugh. It is hoped that both stations will continue their news releases.

From: Commander Amphibious Forces, United States Pacific Fleet (Commander Joint Expeditionary Force)

No man-made obstacles were encountered at the water's edge of the beaches as had been found in previous operations, and this is attributed to the fact that they would not have remained in place. The shifting volcanic sand and surf would have soon covered them up or washed them away. However, the natural terraces immediately behind the beaches offered a greater barrier than man-made obstacles. Only tracked vehicles could cross them, and even some of these were stalled by craters in the sides of these terraces. Another reason was that all beaches could be well covered by fire from the Suribachi Mountain to the south, and the high cliffs overlooking the beaches from the north.

The defense inland was the best yet encountered by this force. In spite of the unusual amount of advance information and the excellent large scale photo coverage of Iwo Jima, it was impossible to accurately determine the exact strength and magnitude of the defensive installation. A great majority of the installations were located and printed on the final installation map. Those that were not located could not be located from photographs. This is attributable to the most effective camouflage, both natural and artificial, and the fact that many of the defenses were in caves which were most difficult to locate.

The salient features of the defensive installations were the unusually large number of intricate, heavily protected, interconnected series of underground shelters; an unprecedented number of heavy mutually supporting pillboxes; skillfully placed artillery, mortars, and antiaircraft guns; extensive and effective use of land mines, including those improvised from artillery shells and bombs; and the considerable number of caves in the mountainous and cliff areas which housed supplies, command posts, and artillery, machine guns, and coast defense guns. A good example of the cave system found on the island was found in TA 183L. There were nine entrances to the cave, and the main passageway extended about 800 yards coming out at Minami Village. Two CP's and an aid station had been located there. Many of the above were picked up by aerial photography, but as stated before due to the excellent camouflage

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and vegetation it was impossible to locate them all. Too, the extent and exact nature was estimated, but no definite information was available. As the operation progressed and vegetation was blasted away, these additional ones were located. However, in the case of many mortar positions, they were not actually located until troops had stepped into them or noted them in passing.

Many of these installations could not be knocked out by air, artillery, or naval gunfire due to their nature and location. These had to be subjected by the troops. Considerable difficulty was experienced with the coast defense guns, all of which were in the caves along the cliffs and on the slopes of Suribachi. Several of the ships of this force sustained serious damage from these guns.

Beach mines were found along the first terrace behind the GREEN and RED Beaches. These were placed 4 feet apart in rows. One small field was found on the western beach just south of PURPLE Beach. A considerable number of minefields were reported further inland by the troops.

As a whole, this was the best defended, most well organized, and most difficult objective this force has ever operated against. The location and organization of the defenses was the best tactically yet encountered.

Plans for the operation called for the assignment of an experienced oceanographer to the Underwater Demolition Group, in order that preliminary surf observations could be made at the time of the beach reconnaissance; and that forecasts could be made daily of expected surf conditions in conjunction with the aerological officer. Unfortunately, the oceanographer assigned was evacuated to the rear area due to illness just prior to departure of the Underwater Demolition Group for the objective, and no replacement was available.

Aerial observations of the beach conditions, including surf, were made by three observers specially trained for this work. All three were aerological officers who had had a short course in study of waves and surf at Scripps Institution of Oceanography; special training in aerial observation; and previous experience in the Palau operation. Their reports, as regards surf, were of little assistance in forecasting. The probable reasons for this are, (1) the sea was relatively calm when the observation flights were made, so that the surf was negligible; (2) the character of the beach was such that only a general idea of the surf could be obtained by an aerial view.

The scarcity of reports, and uncertainty of the analysis of the weather map in this area made accurate and dependable forecasts of swell impossible. Within the limits of the weather map, however, such forecasts were made, and the aerological officer on the Attack Force Commander's Staff kept beachmasters and landing forces advised of expected surf conditions.

From: Commander Task Group 51.1 (Commander Transport Squadron 11 and Commander Transport Division 31)

An examination of the excellent beach intelligence photographs available indicated that, if either chop or moderate swell existed LSM and LCVP would, though manned by experienced personnel, be of limited use. The abrupt steep gradient type of beach causes high surf which will poop boats, although skillful handling may prevent broaching.

MAGNETIC PROPERTIES OF SAND ON IWO JIMA

From: A Report by the Military Geology Unit, United States Geological Survey--Giving Results of a Test of a Sample of Beach Sand

This is a clean, course, dark-colored sand.

About 40 percent of the grains are picked up with a strong (Alnico) hand magnet.

Almost all grains are of the same material and composed of varying proportions of the following:

  1. Dark brown glass (about one-half to two-thirds).

  2. Andesine crystals (about one-quarter to one-third). A few grains have idiomorphic boundaries. All include glass, some an extraordinary large amount. Some grains have a very peculiar scalloped outline.

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  1. Augite (about one-eighth).

  2. Magnetite (accessory grains). Rather abundant in fairly large grains (none occur as dust-like grains). Magnetite gives the sand its strong magnetic property.

  3. Ilemnite. A very few grains of ilmenite are included in the magnetite.

In addition to the material described above a few sand grains of a different character are found, namely, a very fine-grained glass andesite with the plagioclase present as microliths. These grains are better rounded than the rest of the material and have clearly been transported farther than the bulk of the material which probably came from the immediate vicinity of the beach.

Conclusion: The magnetite in the rock is the cause of the difficulty encountered in using the SCR-625 land mine detector.

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