Amphibious Assault

Photograph of landing craft approaching Iwo Jima

Amphibious assault is the military operation of landing troops on a shore under fire. It is rightly regarded as one of the most difficult of military operations. Defending positions usually have good cover and excellent fields of fire against boats approaching the shore and troops on the beach. On the other hand, the assaulting troops lack cover and are unable to move quickly while coming ashore. Logistics is a major difficulty for the attacker, since the usual port facilities for unloading ships are absent. Until the beachhead is secured, there is no rear area in which to deploy artillery and other supporting arms.

During the First World War, the Allies suffered a costly defeat in their amphibious assault on Gallipoli, a strategy for which Churchill took much of the blame. Most military strategists in the interwar years assumed that a successful amphibious assault against determined opposition was impossible. A prominent exception was the U.S. Marine Corps, which was looking for a new mission and found it in the Navy's contingency plan for war with Japan, Plan Orange. This plan called for an advance across the central Pacific to the Philippines, which would require the capture by amphibious assault of defended islands in the Mandates.

The amphibious doctrine of 1919 was primitive. The Navy envisioned loading 50-foot motor launches with as many Marines as they could carry, then towing them toward shore while the warships fired a few shells at enemy positions. The Navy did not consider an effective bombardment to be a realistic hope because of the belief that "a ship's a fool to fight a fort", meaning that warships should not operate in range of fortified coastal artillery, and also because the ships would have to carry both armor-piercing shells for use against enemy warships and high explosive shells for use in the bombardment. There was also the problem of finding suitable landing sites, since motor launches had a very wide turning radius.

However, in 1931 three Marine majors, Charles Barrett, Pedro Del Valle, and Lyle Miller, began work on the Tentative Manual for Landing Operations. On 14 November 1933, all classes at Quantico were suspended and all students and instructors were put to work refining the manual. The Marines closely studied the Gallipoli campaign and concluded that it could have succeeded with the right landing doctrine. The Tentative Manual for Landing Operations, which has been characterized as "as much a catalogue of problems that would have to be solved in an amphibious assault as a guide to its execution" (Spector 1985). However, in many respects it proved remarkably prescient in its assessment of the requirements for a successful amphibious assault. Some of the problems it identified were that naval artillery fired on too flat a trajectory for land targets and had only limited reserves of ammunition; the proper use of air support was unknown; and ordinary ship's boats were completely inadequate to transport attacking troops from ship to shore. The last problem was in many respects the key to the situation.

Landing Craft. Craft for transporting troops to shore must be sufficiently shallow in draft to be able to beach high up on the shore; they must enable the troops to disembark very quickly; and they must be able to easily back off the beach to retrieve subsequent waves. Ordinary ship's boats do not meet these requirements. The United States found a solution in the LCVP or Higgins boat, based on marsh boats developed by Andrew Higgins of Louisiana. These boxy, flat-bottomed craft were driven by a recessed drive shaft and propeller that was protected from grounding damage and had a bow ramp that could be dropped in very shallow water to allow troops to disembark rapidly. They thus met all the requirements previously outlined for a landing craft.

Experience in early operations made it clear that larger landing craft would be required, capable of carrying vehicles, including tanks to support the landing. This led to the development of the LST, an oceangoing ship with bow doors that could land surprisingly high up on a beach and disembark vehicles.

Another development was the LVT or amtrack, an amphibious tracked vehicle that could swim to shore and crawl out of the water and on inland. These were lightly armed and armored and proved to be the key to successful amphibious assaults in the central Pacific. At Tarawa, where conventional landing craft were hung up on the reef and their passengers massacred, the amtracks were able to crawl across the reef and turn the tide of the battle. It was unfortunate that only a relatively small number of these craft were available. Later assaults were much more lavishly equipped with LVTs, but these relatively expensive craft were never able to completely displace simpler types of landing craft.

Another important development was the LSD, an oceangoing vessel with a large dock in the stern that could be flooded by ballasting down the vessel. This dock could hold a number of smaller landing craft and launch them much more easily than a conventional transport, which had to lower its landing craft from davits and load the men from cargo nets. By the end of the war, it was becoming clear that the ideal amphibious fleet was spearheaded by LSDs carrying amtracks and supported by LSTs or other ships capable of landing tanks and vehicles.

Sea Lift. In late 1943, the sea lift requirements for a reinforced division of about 20,000 men (three regimental combat teams, their ammunition, 1500 vehicles, and supplies) was reckoned at 12 attack transports, three attack cargo ships, and one LSD. Each regimental combat team was carried by a transport division consisting of 4 attack transports and an attack cargo ship. A corps headquarters and supporting units required four attack transports and four attack cargo ships, in addition to those required for its divisions.

If the distance was short then some of the sea lift could take the form of short-range beaching vessels such as LSTs and LCIs.

The U.S. Army made some important contributions to amphibious doctrine in the area of logistics, at which it excelled. During the invasion of Attu, 7 Division pioneered the use of palletized supplies. The use of pallets caused friction between Holland Smith and Ralph Smith in the rehearsal for Makin, since the Marine commander wanted a full rehearsal and the Army commander feared this would require extensive reloading of the pallets. Eventually a compromise was worked out, but this likely contributed to later tension between the commanders leading to the "War of the Smiths."

Fire Support. Until a beachhead could be secured in which artillery could be deployed, the troops ashore had to be supported by naval firepower. This proved problematic. Warships carry a limited number of shells and fire them on much shallower trajectories than is usually the case with land artillery. It was found at Tarawa that naval bombardment was not nearly as effective as had been hoped. Experiments at Kahoolawe in Hawaii showed that naval bombardment had to be highly accurate plunging fire to have much effect on Japanese fortifications. The Navy's older battleships, including survivors of the attack on Pearl Harbor, became the heart of the amphibious bombardment force, freeing the more modern battleships to escort carrier task forces.

Another source of fire support was the landing craft themselves, which were fitted with increasingly heavy armament. In addition, a number of amtracks were fitted with tank turrets ("amtanks") to provide direct fire support, while other landing craft were fitted with large batteries of rockets that could deliver a devastating barrage.

Air support was an important component of fire support from the very start. However, it took some time to learn how to make air support effective. Fighter-bombers like the Corsair were fitted with rockets and bombs, including napalm bombs, as the war progressed. Air support typically was flown from task units of escort carriers, leaving the fleet carriers free to provide distant cover or carry out supporting raids deep into Japanese territory.

Towards the end of the war, fire support become so effective that the Japanese largely abandoned efforts to hold at the water's edge, and turned to defense in depth, out of range of direct naval gunfire. This pattern was first seen at Peleliu. The Japanese plan in the event of an invasion called for the defenders to stay well back from the beaches until the Allies were ashore, then engage them as closely as possible to nullify the Allied long-range firepower.

Intelligence. A successful opposed landing must have adequate intelligence about the target. The attacker must know where his landing craft can cross any reef and successfully beach themselves, and the locations of enemy fortifications must be pinpointed for the pre-landing bombardment. Intelligence was obtained by photoreconnaissance aircraft, by submarines taking photographs through their periscopes (though these were of limited value), and by underwater demolition teams scouting the reef and beach while demolishing natural and man-made obstacles with explosives. Photoreconnaissance aircraft took both high-altitude stereoscopic pairs of photographs for mapping purposes and oblique photographs from very low altitude that gave a ship's-eye view of the target.

Japanese Landing Operations. The Japanese avoided opposed landings whenever possible. During the Centrifugal Offensive this was almost always possible, because the Japanese had command of the sea and Allied forces were stretched very thin. The only attempts by the Japanese to land under heavy fire both took place at Wake; the first attempt failed before the assault troops had even gotten into their boats, and the second succeeded only with heavy casualties in spite of overwhelming numbers and massive air and fire support.

In spite of a doctrine of avoiding opposed landings, the Japanese made some important innovations in amphibious craft that often closely paralleled those of the Allies. Indeed, the Japanese were well ahead of the Allies in some respects when war broke out. For example, the Japanese had already developed landing craft, such as the Daihatsu, that strongly resembled the LCVP in form and function. The Japanese Army and Navy published an Outline of Amphibious Operations that predated the Tentative Manual for Landing Operations. The Japanese also developed some innovative landing ships, such as the Akitsu Maru, which could carry and launch both landing craft and aircraft. These prefigured some of the features of postwar amphibious assault ships.

Invasion Defenses

The Japanese Army remained preoccupied with the Soviet threat until September 1943, when the Army schools finally switched their emphasis to countering American tactics. Prior to that time it was assumed that the Navy was responsible for fighting the Americans. Emphasis was originally put on counterattacks against the beachhead, but when this failed (particularly at Peleliu, where a Japanese tank-infantry counterattack was cut to ribbons while a defense in depth in the high terrain held up the Americans for a month) the Army was force to change tactics. The Army issued its "Esssentials of Island Defenses" in August 1944, which called for a mobile defense anchored to strongpoints from which local counterattacks could be launched. Fortifications, dispersal, concealment, and fighting spirit were emphasized. In October a new draft was released that abandoned defense at the water's edge and called for defense in depth. However, it proved difficult to overcome a generation of training that emphasized cold steel and offensive spirit, and the Emperor himself questioned why the commander at Okinawa abandoned the airfields to the Americans rather than defend the beaches.

References

Morison (1951, 1953)

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