U.S. Navy's amphibious operations in WWII and the development of UDTs ("Navy Frogmen")** the UDTs grew directly out of the Navy's need to prepare beaches and obstacles for amphibious assaults.
The U.S. Navy played a central role in moving, landing, supplying, and protecting enormous numbers of troops during amphibious assaults. Major operations included:
The basic problem was deceptively difficult: ships had to get troops and equipment onto an often hostile shoreline while dealing with mines, underwater obstacles, reefs, beach defenses, surf, tides, and enemy fire.
The Underwater Demolition Teams (UDTs) were created to solve the underwater portion of that problem.
Their predecessors emerged in 1943, particularly from the Navy's Seabee and combat-demolition organizations. The teams conducted reconnaissance of beaches and obstacles and, when necessary, demolished obstacles so landing craft could reach the beach.
At Tarawa in November 1943, underwater obstacles and reefs demonstrated how dangerous the approach to a defended beach could be. Navy personnel subsequently developed specialized teams capable of operating in the water ahead of the main assault force.
By the later Pacific campaigns, UDTs were conducting missions such as:
They generally operated ahead of the main amphibious force, often under extremely hazardous conditions.
The term "frogmen" became associated with these underwater demolition personnel because of their diving equipment and underwater work.
However, it's worth distinguishing WWII UDTs from the later popular image of a modern Navy SEAL.
WWII UDTs ≠ modern SEALs.
The UDT mission was principally underwater reconnaissance and demolition in support of amphibious operations. They were not originally a general-purpose special-operations force in the modern sense.
The lineage is roughly:
WWII combat demolition / Seabees → Underwater Demolition Teams (UDTs) → UDT/SEAL teams → modern Navy SEALs
The UDTs became particularly important in the Pacific because many Japanese-held islands presented exactly the kind of problems they were designed to solve: coral reefs, shallow water, beach obstacles, mines, and heavily defended landing areas.
During the September 1944 invasion of Peleliu, Navy amphibious forces had to get the Marines across the reefs and onto heavily defended beaches. UDTs conducted reconnaissance and demolition work in advance of the landings, helping identify and clear obstacles in the approaches.
Peleliu also illustrates the limits of amphibious technology: even with extensive naval preparation, the Japanese defenses—particularly their mutually supporting positions inland—made the operation extraordinarily costly.
