Amphibious warfare
The Inter-Service Training and Development Centre was established at Fort Cumberland, near Portsmouth in 1938, and brought together representatives from the Royal Navy, Army, and Royal Air Force convened with the portfolio of developing methods and equipment to use in Combined Operations.
The Centre examined certain specific problems, including craft for landing tanks, beach organisation, floating piers, headquarters ships, amphibian tanks, underwater obstacles, the landing of water and petrol and the use of small craft in amphibious raids By the end of 1939 the ISTDC had codified a policy for landings, and defended it at Staff College discussions. Operational experience during the Second World War introduced modifications to this landing policy, but it was essentially the policy used in the Torch and Husky landings four years later.

The essential shape of this landing policy is described by Bernard Fergusson in The Watery Maze, 
The system provided for an approach under cover of darkness in fast ships carrying special craft; the craft being sent ashore while the ships lay out of sight of land; small-craft smoke and gun protection while the beachhead was seized; the landing of a reserve; the capture of a covering position far enough inland to secure the beach and anchorage from enemy fire; the bringing in of ships carrying the main body; and finally the discharge of vehicles and stores by other craft specially designed to do so directly on to beaches. And in all this it was important to achieve tactical surprise.
Among the many tactical innovations introduced by the centre, codified in the Manual on Combined Operations and the Standard Naval Bombardment Code, was the use of Floating Piers (pontoons) to bridge the water gap, the creation of Smoke Generating devices to obscure the assault and the use of infrared directional beacons for landing accuracy. The centre also played a role in the development of the first specialized landing crafts, including the Assault Landing Craft, the Mechanized Landing Craft (LCM(1)), the Landing Craft Tank (Mk. 1), Support Landing Craft LCS(1), LCS(2) and Landing Ship Infantry.
Divisional-sized amphibious landing exercises were carried out by the British Army in the 1930s.

United States: In contrast to the British attitude, the U.S. military, especially the Marine Corps remained enthusiastic at the possibilities of amphibious warfare. The Marine Corps was searching for an expanded mission after World War I, during which it had merely been used as a junior version of the Army infantry. During the 1920s, it found a new mission—to be a fast-reacting, light infantry fighting force carried rapidly to far off locations by the US Navy. Its special role would be amphibious landings on enemy-held islands, but it took years to figure out how to do that. The Mahanian notion of a decisive fleet battle required forward bases for the Navy close to the enemy. After the Spanish–American War the Marines gained the mission of occupying and defending those forward bases, and began a training program on Culebra Island, Puerto Rico.
As early as 1900 the General Board of the United States Navy considered building advance bases for naval operations in the Pacific and the Caribbean. The Marine Corps was given this mission in 1920, but the challenge was to avoid another disaster like Gallipoli. The conceptual breakthrough came in 1921 when Major "Pete" Ellis wrote Advanced Base Operations in Micronesia a secret 30,000-word manifesto that proved inspirational to Marine strategists and highly prophetic. To win a war in the Pacific, the Navy would have to fight its way through thousands of miles of ocean controlled by the Japanese—including the Marshall, Caroline, Marianas and Ryukyu island chains. If the Navy could land Marines to seize selected islands, they could become forward bases.

Ellis argued that with an enemy prepared to defend the beaches, success depended on high-speed movement of waves of assault craft, covered by heavy naval gunfire and attack from the air. He predicted that the decisive action would take place on the beach itself, so the assault teams would need not just infantry but also machine gun units, light artillery, light tanks, and combat engineers to defeat beach obstacles and defenses. Assuming the enemy had its own artillery, the landing craft would have to be specially built to protect the landing force. The failure at Gallipoli came because the Turks could easily reinforce the specific landing sites. The Japanese would be unable to land new forces on the islands under attack.
Not knowing which of the many islands would be the American target, the Japanese would have to disperse their strength by garrisoning many islands that would never be attacked. An island like Eniwetok in the Marshall Islands, would, Ellis estimated, require two regiments, or 4,000 Marines. Guided by Marine observer aircraft, and supplemented by Marine light bombers, warships would provide enough firepower so that Marines would not need any heavy artillery (in contrast to the Army, which relied heavily on its artillery). Shelling defended islands was a new mission for warships. The Ellis model was officially endorsed in 1927 by the Joint Board of the Army and Navy (a forerunner of the Joint Chiefs of Staff).
However, actual implementation of the new mission took another decade because the Marine Corps was preoccupied in Central America and the Navy was slow to start training in how to support the landings. The prototype advanced base force officially evolved into the Fleet Marine Force (FMF) in 1933. In 1939, during the annual Fleet Landing Exercises, the FMF became interested in the military potential of Andrew Higgins's design of a powered, shallow-draught boat. These LCVPs, dubbed the 'Higgins Boats', were reviewed and passed by the U.S. Naval Bureau of Construction and Repair. Soon, the Higgins boats were developed to a final design with a ramp, and were produced in large numbers.

Second World War: By the Second World War tactics and equipment had moved on. The first use of British landing craft in an opposed landing in the Second World War, saw the disembarkation of French Foreign Legionnaires of the 13th Demi-Brigade and supporting French Hotchkiss H39 tanks on the beach at Bjerkvik, eight miles (13 km) above Narvik, on 13 May during the Norwegian campaign.
The first major and successful amphibious operation was Operation Ironclad, a British campaign to capture Vichy French-controlled Madagascar. The naval contingent consisted of over 50 vessels, drawn from Force H, the British Home Fleet and the British Eastern Fleet, commanded by Rear Admiral Edward Neville Syfret.

The fleet included the aircraft carrier Illustrious, her sister ship Indomitable and the aging battleship Ramillies to cover the landings. The first wave of the British 29th Infantry Brigade and No. 5 Commando landed in assault craft on 5 May 1942, follow-up waves were by two brigades of the 5th Infantry Division and Royal Marines. Air cover was provided mainly by Fairey Albacore and Fairey Swordfish torpedo bombers which attacked Vichy shipping.
Purpose-built landing craft were among the vessels used at the evacuation from Dunkirk (Operation Dynamo) and an amphibious operation was tried out at Dieppe in 1942. The operation proved a costly failure, but the lessons, hard learned, were used later. Many small-scale operations were conducted by the Allies on the Axis-held coast of Europe, including raids on the Lofoten Islands, St Nazaire and Bruneval.

Specialized infantry landing craft: In the run up to World War II, many specialized landing craft, both for infantry and vehicles, were developed. In November 1938, the Inter-Service Training and Development Centre proposed a new type of landing craft. Its specifications were to weigh less than ten long tons, to be able to carry the thirty-one men of a British Army platoon and five assault engineers or signallers, and to be so shallow drafted as to be able to land them, wet only up to their knees, in eighteen inches of water. All of these specifications made the Landing Craft Assault; a separate set of requirements were laid down for a vehicle and supplies carrier, although previously the two roles had been combined in the Motor Landing Craft.

J. S. White of Cowes built a prototype to the Fleming design. Eight weeks later the craft was doing trials on the Clyde. All landing craft designs must find a compromise between two divergent priorities; the qualities that make a good sea boat are opposite those that make a craft suitable for beaching.
The craft had a hull built of double-diagonal mahogany planking. The sides were plated with "10lb. DIHT" armour, a heat treated steel based on D1 steel, in this case Hadfield's Resista 1⁄4".

The Landing Craft Assault remained the most common British and Commonwealth landing craft of World War II, and the humblest vessel admitted to the books of the Royal Navy on D-Day. Prior to July 1942, these craft were referred to as "Assault Landing Craft" (ALC), but "Landing Craft; Assault" (LCA) was used thereafter to conform with the joint US–UK nomenclature system.
The Landing Craft Infantry was a stepped up amphibious assault ship, developed in response to a British request for a vessel capable of carrying and landing substantially more troops than the smaller Landing Craft Assault (LCA). The result was a small steel ship that could land 200 troops, traveling from rear bases on its own bottom at a speed of up to 15 knots.