D-Day was not the first amphibious assault of WWII. The Allies had carried out no less than eight major amphibious assaults in Europe between 1940 and 1944, from Narvik in 1940 to Anzio in January 1944. Many more had been fought in the Pacific, or as small raiding actions in Europe. The entire Allied force was aware of how tricky an amphibious assault could be; how risky it was, but also how likely it was to succeed - only one assault, Dieppe, had been a failure.

Gallipoli did loom large in Royal Navy amphibious doctrine in the interwar period. As the only major example of an amphibious assault to actually be carried out during WWI, commanders had to work out how to avoid making the same mistakes, and running into the same problems. At Gallipoli, the troops at many beaches had been pinned down shortly after disembarking by machine guns and barbed wire, while the ships offshore could not effectively break the deadlock thanks to communication difficulties. The troops had been transported to the beach in slow, vulnerable boats, some towed by motorboats, others rowed. The troops could not then exploit what gains they had managed to achieve, as they lacked their own organic artillery and other supporting arms. Several solutions were tried, with some successful, and others found wanting.

The first suggestion was that the initial landings should take place in the early hours of the morning; this would give the initial assault waves the cover of darkness to secure the beach for more troops to be landed after dawn. The assumption was that the defenders would not be able to see the first waves, at least not until they had hit the beach. A number of exercises proved this theory wrong. An exercise carried out at Kasid Bay, India in 1925 found that, at both the assaulted beaches, the defenders could, even at night, see the boats fifteen minutes before landing. A further exercise, carried out off the Yorkshire coast in 1934, found similar issues, albeit this time with an attack at dawn, covered by twilight. It was concluded that a daylight attack, which would greatly simplify navigation, would be just as effective, especially if supported by smoke screens and naval gunfire.

Tanks also provided another way to help the troops off the beaches. Their armour and guns would allow them to crush most fixed defensive positions, but also made it difficult to transport them to the beach. The first attempt at landing tanks as part of an amphibious operation came with the cancelled Operation Hush in 1917, a plan to land British troops on the Flanders coast. This operation had little effect on further planning, but it was still clear that tanks would make an amphibious assault much easier. Serious thought was given to the problem from 1920, and a series of 'Motor Landing Craft' were constructed. These also helped to solve the problem of getting the troops ashore swiftly, and of landing their equipment and supplies. There were also trials with amphibious tanks, but most failed; the failure showed the need for even more landing craft. The 1934 Yorkshire exercise tested the ability of landing craft to transport troops and supplies. It concluded that they were capable of doing so, but that large amounts of craft were necessary; albeit smaller ones than tested in the exercise.

The final problem to be solved was how to support the troops ashore. The Royal Navy was relatively well practised in shore bombardment; unfortunately, it was less experienced in supporting troops, as was the RAF. The 1925 Manual of Combined Operations laid out the need for radio communications between observers ashore, spotting planes, and the ships offshore. It suggested the need for a combined headquarters ship, but did not explain in detail which ship should be chosen, or what radios were needed. In 1933, an exercise ashore at Sandhurst determined how troops could communicate with ships offshore, concluding that things would be very difficult. Battleships did not have enough radio equipment, let alone the necessary modern radios. Liners could be used, but were vulnerable, and needed extensive conversions. The 1934 exercise put these conclusions to the test, finding that battleships worked well as headquarters ship. It also helped establish effective procedures for forward observers.

By the time of WWII, the Royal Navy combined the experiences from these pre-war exercises with those gathered by the comparable American exercises, as well as with observations of Japanese operations in China, to produce a relatively effective amphibious doctrine. The failures of Gallipoli had been solved. This was put to the test multiple times. At Narvik, the RN was able to successfully land troops, including tanks, albeit against light opposition. In 1941, small-scale raids against the Norwegian Coast, such as Operations Claymore, Anklet and Archery, honed the doctrine for further operations. The raid on Dieppe in 1942 was a major failure, but showed the need for modifications to the doctrine. At Dieppe, insufficient offshore support had been provided, and the Allies had made several intelligence failures, failing to properly determine that the beach was unsuitable for tanks and that the target was heavily fortified. It also showed that an assault directly on an enemy-held port was impossible. Amphibious operations in the Mediterranean, from Torch through to Anzio, helped to integrate British and American doctrine and procedures. By the time of D-Day, the Allied commanders could be sure that Gallipoli was not to be repeated. The troops were aware that the landing was to be dangerous - there were some fears that the first wave could be wiped out. Cornelius Ryan's The Longest Day mentions some troops being told that casualties in their units could be as high as 84%, or 60% in armoured units. These fears were generally not spread by well-informed officers.