Why interwar amphibious plans lacked landing ships

Interwar navies prepared plans to convert merchant vessels into troopships or auxiliary warships, but they did not build the specialist fleet later associated with large amphibious campaigns. Japan possessed only a few purpose-built vessels. The Royal Navy and United States Navy had no comparable inventory because recent requirements had not forced one. An amphibious force needs a combined system of ships, landing craft, and troops rather than any single component. Landing craft were expected to ride on ordinary merchant-ship davits, so interwar plans still depended on improvised shipping for deployment.