 On 9 April, all three regiments of the 40th Division pushed east into the rugged, mountainous interior of the island.The Japanese resisted stubbornly, aided by booby-trapped terrain, defended their fortified positions by day, and conducted harassing attacks at night.Soon, the 40th Division started using small infiltrating units to creep past tank traps and minefields, then scrambled uphill across open fields of fire to attack Japanese positions.