Even before the Marianas were secure, the Seabees were building airfields for the big B-29 heavy bombers. These bombers started flying from Guam, Saipan and Tinian in August 1944 against precision targets in Japan.
Their flight plan called for them to overfly a volcanic mountainous atoll called Iwo Jima in the Bonin Islands. The mountain, called Mount Suribachi, had heavy antiaircraft emplacements and large numbers of fighters based there. B-29s were being lost on the way in and the way out, due to lack of fighter cover and an emergency landing strip. Iwo's fighters were also a huge problem.
The decision was made; Iwo Jima had to be taken. Photoreconnaissance showed that Mount Suribachi was being dug in with gun emplacements and pillboxes above and below ground. Constant air bombardment began and lasted for 72 days.
In September 1944, soon after the fall of Saipan, Lieutenant General Tadamichi Kuribayashi studied the map of the Pacific in the Imperial Japanese Navy's Headquarters in Tokyo. He could see that the war was not going well for Japan; it was only a matter of time before the Americans were in striking distance of Japan itself. He realized that he had to inflict as many casualties on the Americans as he could on Iwo Jima. He told his wife not to expect his return. The only advantage he held was that the US High Command underestimated the number of defenders on Iwo Jima.
He constructed an elaborate system of bunkers connected by tunnels. He forbade the charges that had characterized the fighting on other islands, and ordered each men to kill ten of the enemy before he himself was killed. The tunnels were very hot due to the active volcano and medicine and food was in short supply.