A labyrinth of rocky walls, hills, crags, forests, and tunnels make up Peleliu Island. The Japanese had fortified the island with formidable defenses. Japanese forces were able to move covertly between battle zones thanks to a network of more than 500 caves and tunnels.
It was very important that Navy UDTs, also known as “frogmen,” were used to prepare for the invasion.
Unless the amphibious craft could get over the reef, avoid the mines, navigate the concrete anti-boat obstacles, the coral heads, and boulders, it (the invasion) was doomed to failure, according to an article by Toni L. Carrell, Ph.D., Chief Scientist and Principal Investigator, Ships of Discovery and Discovery Research. All the planning involved UDT reconnaissance.
Prior to the Peleliu assault, UDT 10 conducted an invasion beach scouting mission on the USS [United States Ship] Burrfish. The data collected in August 1944 showed a variety of cement tetrahedrons, a dual row of timber frames 75 yards from shore, barbed wire, horned mines, and, most critically, that the reef was only covered by two feet of water at low tide in some spots.
UDTs 6 and 7 were deployed three days before D-Day along the invasion beaches to clear the way but, more crucially, to rip huge ramps out of the coral for the amphibious craft. In addition to the fact that their task was hazardous and taking place in broad daylight, navy fire support aircraft flew overhead, and occasionally sniper and machine gun fire were directed at the unarmed swimmers in the narrow lagoon from the shore. The night before the assault, UDTs slithered ashore to destroy concrete cubes, posts, rock cribs, and barbed wire. They also put buoys off the reef to designate the recently blown tunnels.
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The amphibious assault strategy to take Peleliu is also described by Carrell, who says, “The new plan envisioned five hypothetical parallel lines offshore where different elements of the working group could stage with their ships and troops before the assault. The large ships and transports travelled the farthest, at 18,000 yards. The troops were being transported in LVTs (landing vehicle tracks) in the massive holds of the LSTs, which arrived next. The little LVTs (also known as amtracs) boarded at 6,000 yards from land when the LSTs opened their bow doors.
The fourth line was 4,000 yards from the coast, taking 30 minutes to get there. All of the assault waves were to gather here and form groups to face their designated beaches. The last line before the reef was at 2,000 yards and 15 minutes from shore, where the amtracs turned around after transporting the assault waves to the beach and where the following groups of men and supplies were transported from small boats to the amtracs. It was up to the troop-carrying amphibious flotilla to cross the reef and get to land when they got to the last line, which was 1,000 yards away.
Submarine chasers, patrol boats, and Higgins boats were in regular radio contact, forming up the waves, erecting signal flags, and policing the small fleet at each line. In order to eliminate beach fortifications and support the landings, armored LVT(A)s (amphibian tanks) were deployed ahead of the first waves of soldiers.
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At the 1,000-yard line, LCI(G) (landing craft, infantry, gunboats) with rocket launchers waited offshore and pounded the LVT(A)s with cover fire. Naval gunfire pounded the island from above, and planes attacked and strafed it. It took careful timing and coordination to complete the difficult landing technique.
On September 15, 1944, the 1st Marine Division started its beach landings on the island at 0830 (military time), following days of intense US naval and aerial bombardment on Peleliu. Beaches White 1 and 2 saw the arrival of the 1st Marine Regiment, while Orange 1 and 2 saw the arrival of the 5th Marine Regiment, and Orange 3 saw the arrival of the 7th Marine Regiment.