Harry Truman. FDR was a virtual corpse by February 1945. Harry Truman may have been forced to take an active role in global politics and the War. The original stated mission for the invasion of Iwo Jima was to depress Japanese fighter interference of the non-stop daylight raids by B-29 bombers on the defeated country. The problem was that the US controlled the sky and raids from the tiny airfield could be eliminated by destroying the field with B-29 bombers. Naval gunfire on Iwo Jima was less than half the time the Marines requested. The Navy had nowhere else to go so it can be reasonably assumed that the intent was not to kill the Japanese defenders but to test their effectiveness of the Japanese on an island that was considered to be a part of the original Japanese empire in order to gauge the impact of a landing on the mainland. Truman's hatred of the Marines since his WW1 days was evident in his post WW2 effort to decrease the Marines to a token force. Even if Marines failed in their effort and abandoned the Iwo Jima mission it would still be a win for Truman and and most importantly a vindication of the scientists who were anxious to release the genie out of the nuclear bottle. Marines lost a staggering 6,000 in a month on the stinking island. FDR gsped when he read the casualty figures but the bean counter had his answer and all he had to do was revise the original mission. The reason Marines invaded Iwo Jima would be spun as an effort to secure a landing for crippled B-29's and B-29's were ordered to divert from the Philippines and take the long hazardous flight to a tiny island so Truman could say the loss of 6,000 Marines was worth it.
You might have made a case for incompetency on the navy part if you had brought up the marine invasion of the Palau islands. Those islands were not needed after it was decided to invade Leyte. Marine casualties were 6526. Marine decisions were made by the navy, not Truman. Should America have used gas warfare to take Iwo, it had been suggested? The B29's were based in the Marianas in the Central Pacific, not the Philippines. The Japanese air force on Iwo caught the B29's on their way to Japan and then again on their way home. A bonus was the saving of crippled B29's on their way home, and those were not only men but airplanes saved.