That left the LSD, which became the basis of the post-World War II U.S. amphibious force. The San Antonio-class LPD USS Arlington is recognizably a direct descendant of the earlier LSD, though a great deal has changed. The most dramatic change in amphibious practice is the use of helicopters. As in the past, the key issue was how quickly the landing force could build up on the beach. The U.S. Marines realized that helicopters could bring troops and light equipment ashore far more quickly than landing craft – and far less vulnerably. By the early 1960s they had a doctrine of vertical envelopment, in which troops and light weapons would move in by helicopter to secure a perimeter around a beachhead. Heavy equipment, including armor, still had to come over the beach. The Marines did not want to rely entirely on helicopters to bring in their troops, so they continued to provide an alternative seaborne method, using tracked amphibious vehicles, descendants of the World War II amtracs.

The well deck ships of the 1960s included large ships with full-length helicopter decks, the Tarawa-class amphibious assault ships (LHA – amphibious helicopter assault). By the 1970s they formed a complete amphibious force, with well decks adapted to the one major surviving World War II amphibious craft, the beaching craft that carried vehicles from the dock ships to the beach.

Beginning in the 1960s, the United States built a 20-knot amphibious force specifically to support non-nuclear operations in limited wars such as Vietnam. For example, in 1965 Marines defeated an enemy force by landing in its rear. This new force was adapted to helicopter assault. New amphibious ships mixed cargo, vehicles, and personnel. In order to land troops by helicopter, they had at least to have helicopter decks, though some initially had no hangars. The new generation was designated LPD, for amphibious (L, landing), personnel (P), and dock (D). the San Antonio-class LPDs such as Arlington are the latest incarnation of this idea.

The well deck ships of the 1960s included large ships with full-length helicopter decks, the Tarawa-class amphibious assault ships (LHA – amphibious helicopter assault). By the 1970s they formed a complete amphibious force, with well decks adapted to the one major surviving World War II amphibious craft, the beaching craft that carried vehicles from the dock ships to the beach.

This new-generation force was still limited in one important way. An enemy knew that craft could not beach on a very flat beach, because they could not get close enough to dry land when their ramps went down. An entire unit in the Office of Naval Intelligence kept files on the world’s beaches so that amphibious commanders could know where they could land, but the beaching limit was fairly well known, and an enemy commander could follow the same logic. Too, the beaching craft and the companion amphibious vehicles (amtracs) were quite slow, with maximum speeds of 10 knots or less. The ride was no better than in World War II; troops could stand no more than an hour of it. To get to within launch distance of an objective ashore, an amphibious group had to make a long approach, giving an enemy warning.

An LCAC is shaped very differently from the earlier beaching tank carrier. Existing well deck ships could accommodate it, but the numbers were far too small. In effect the LCAC demanded a new generation of amphibious ships corresponding to existing ones, but with differently-shaped docks.

Things changed once more in the 1980s. The Maritime Strategy espoused by the U.S. Navy was designed to impose maritime pressure on a Soviet Union attacking Western Europe to compel it to assign forces to defend its periphery. The threats involved had to be plausible. They included amphibious assaults on Soviet-held territory, such as Jutland (in Denmark) after an initial Soviet conquest. The ideal was leverage: a relatively small amphibious force could, it was hoped, tie down multiple Soviet divisions, a process called virtual attrition. Those divisions could not attack NATO forces in Central Europe. In much the same way, the U.S. Pacific Fleet could help convince the Soviets that they had to maintain large forces on the Chinese border, for fear of a U.S.-backed Chinese attack into Siberia. The more uncertain the Soviets were as to where the Marines might land, the more divisions they would have to spread along the Danish (or other) coast to deal with that threat. Divisions ashore were far less mobile than divisions afloat.

To make the greatest possible uncertainty, the Marines could not afford to be held to the usual beaching limits. They also wanted to be able to mount the assault from beyond the horizon, because that too would delay the defenders. Helicopters could certainly fly from well beyond the horizon, but the landing force would need heavy equipment and armor, particularly because it would be facing an armored enemy force. The Marines found their solution in the current air-cushion landing craft (LCAC). It can use the widest imaginable range of beaches, and it is far faster (at about 40 knots) than its predecessor. Much higher speed means that the force can remain beyond the horizon without increasing the time taken for each run to the beach.

An LCAC is shaped very differently from the earlier beaching tank carrier. Existing well deck ships could accommodate it, but the numbers were far too small. In effect the LCAC demanded a new generation of amphibious ships corresponding to existing ones, but with differently-shaped docks. The first were the Essex-class LHDs, which replaced the LHAs. The LPD-17 class is the LCAC-shaped LPD.

Arlington combines a well deck with a helicopter pad and a hangar, so in effect she is a self-contained amphibious unit. Normally she would operate with a large-deck ship (LHD) and an LSD (to provide extra cargo space plus a well deck), but she might also operate more autonomously, supporting a small Marine unit. Conversely, the existence of numerous well deck (dock) ships has justified construction of large-deck ships, the latest LHDs, without docks. They supply the helicopters and vertical take-off attack aircraft that work with the well deck ships and their LCACs. The LCACs themselves are expensive, and they are aging. It is not clear that they can be replaced in kind. The Marines had hoped to supplement the LCACs with fast protected amphibious fighting vehicles, but they proved too expensive. Without such vehicles, long-range assault is only possible using helicopters, and that may not be acceptable. Thus the LCAC may be replaced either by a newer version of the slow beaching craft or by a fast planing beaching craft. Either can fit a large well deck. The logic of the well deck will survive, because it is what makes it possible for a fast efficient ship to carry an effective fighting force thousands of miles over any ocean and then land it in fighting condition on a distant shore.

There were several ironies. Not only was Inchon the most logical place to land in the North Koreans’ rear, but the Chinese warned the North Koreans specifically that Gen. MacArthur would probably make an amphibious assault, and that Inchon was a likely place. They were not well pleased that the North Koreans scoffed. For their part, the Chinese imagined that since MacArthur’s divisions had made amphibious landings during World War II, they retained that expertise. In fact virtually none of the amphibious personnel had remained, and the Marines made Inchon possible.

This article was first published in the USS Arlington (LPD 24) Commissioning publication.