The first thing you must understand about The Battle of Peleliu is that the Japanese knew that they could never win it. Victory was never possible, and was never their intention.The Japanese believed that, “if they killed sufficiently huge numbers of the enemy before they themselves were killed, the Americans would eventually sicken of the slaughter and sue for peace.” “Divisional Commander Lt Gen Inoue was told that it was his mission to hold the islands for as long as possible and to kill as many Americans as possible-there were not expected to be any Japanese survivors.”
America had far superior Navy and Air Power, but the Japanese had knowledge of the terrain and the Umurbrogol mountains, which America was unaware of. In photographs taken before the invasion, the mountains were shown as “tree covered uplands”. What the photographs didn’t show were the labyrinth of caves and tunnels underneath those uplands, many of which were natural, some of which had been created by the Japanese. According to Wikipedia, “The Umurbrogol contained some 500 limestone caves, interconnected by tunnels. Many of these were former mine shafts that were turned into defensive positions. Engineers added sliding armored steel doors with multiple openings to serve both artillery and machine guns. Cave entrances were opened or altered to be slanted as a defense against grenade and flamethrower attacks. The caves and bunkers were connected to a vast tunnel and trench system throughout central Peleliu, which allowed the Japanese to evacuate or reoccupy positions as needed, and to take advantage of shrinking interior lines.”
The Japanese spent months preparing the Umurbrogol mountains for the invasion; America was totally unprepared for what this would mean.