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.
Marine planners published 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 lauch 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. If the distance was short then some
of the sealift could take the form of short-range beaching vessels such
as LSTs and LCIs.
Fire Support. Until a beachhead is secured in which artillery can be deployed, the troops ashore must 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. Ironically, the Navy's modern battleships were ill-suited for this kind of bombardment, because their high-velocity rifles were not designed to be fired at the necessary elevation. The older battleships, including survivors of the attack on Pearl Harbor, became the heart of the amphibious bombardment force, since their lower-velocity guns were designed for higher elevation firing and could deliver the needed plunging fire.
Another source of fire support was the landing craft themselves, which were fitted with increasingly heavy armament. In addition, 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 Pelelieu.
Japanese
Landing Operations. The Japanese avoided opposed landings
whenever possible, and 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 were both 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 this respect 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
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.
References
The Pacific War Online Encyclopedia (c) 2006-2008 by Kent G. Budge. Index