Major General William Rupertus, the commander of the 1st Marine Division, predicted that the island would be secured within four days.[10] However, after repeated Imperial Japanese Army defeats in previous island campaigns, Japan had developed new island-defense tactics and well-crafted fortifications, which allowed stiff resistance[11] and extended the battle to more than two months. The heavily-outnumbered Japanese defenders put up such stiff resistance, often by fighting to the death in the Japanese Emperor's name, that the island became known in Japanese as the "Emperor's Island."[12]
In the US, it was a controversial battle because of the island's negligible strategic value and the high casualty rate, which exceeded that of all other amphibious operations during the Pacific War.[13] The National Museum of the Marine Corps called it "the bitterest battle of the war for the Marines".[14]
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^[1] Naval History and Heritage Command, "World War II casualties," citing "The Statistics of Diseases and Injuries. vol.3. Washington: Government Printing Office, 1950." Retrieved 2/10/23. "Invasion of Palau Islands: 1171 killed, 101 died of wounds, 1 died as a prisoner."
^[2]"History of the USMC in World War II vol. IV: Western Pacific Operations p. 285, 797. Retrieved 2/10/23. Total for Peleliu island is 1,252 killed, DOW, MIAPD, and 5,274 wounded. Appendix H lists 1,050 killed, 250 died of wounds, 36 missing presumed dead, and 5,450 wounded, including many of those who later died.
^[3]"History of the USMC in World War II vol. IV: Western Pacific Operations p. 285. Retrieved 2/10/23. Total Palau Group: 542 killed and 2,736 wounded or injured in action.
^[4]"Casualty Report Number II" 8 May 1945, p. 65. Retrieved 2/10/23. 424 killed, 179 died of wounds, 9 missing in action, 2,313 wounded and injured in action
^[5] Naval History and Heritage Command, "World War II casualties," citing "The Statistics of Diseases and Injuries. vol.3. Washington: Government Printing Office, 1950." Retrieved 2/10/23. "Invasion of Palau Islands, 185 killed, 10 died of wounds.
^[6]"The History of the Medical Department of the United States Navy in World War II: The Statistics of Diseases and Injuries, Volume 3" p. 84. Retrieved 2/10/23. 699 total battle casualties - 185 killed and 10 died of wounds = 504 wounded and survived. The Marine Corps source linked above references 11, not 10, who died of wounds.
^<ref>[7]"History of the USMC in World War II vol. IV: Western Pacific Operations p. 179, 253. Retrieved 2/10/23. According to American records, the Japanese lost more than 12,000 dead and 300 prisoners on the 3 main islands, excluding several dozen more who survived the initial battle and were captured or killed later.
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