By early 1945, American forces in the Pacific theater were drawing a relentless band of steel around the Empire of Japan. In a series of bold island-hopping campaigns, our forces had crossed thousands of miles of ocean expanse. B-29 bombers of the 20th Air Force had begun a strategic bombing campaign against the Japanese home islands. U. S. military and naval planners held series of conferences during the Summer and Fall of 1944. They identified the Japanese-held island of Iwo Jima as a primary objective for capture.
Enemy fighters based on Iwo's airfields attacked the B-29s causing significant losses. An early warning station on Iwo was able to alert air defenses as the bomber streams overflew the island enroute to Japan. Most importantly, Iwo was a cornerstone of the inner defensive ring around Japan.
Iwo was also important as a potential American base of operations. Its airfields could be used by fighter planes to protect the B-29s. Also, crippled bombers could land on Iwo for repairs and fuel instead of ditching into the ocean. The capture of Iwo would also eliminate the Japanese early warning station there. Finally, by destroying the Japanese garrison on the island, U.S. forces would eliminate a critical base for defense of Japan.
On 9 October 1944, Fleet Marine Force, Pacific received the warning order from Commander in Chief, Pacific to begin preparation for the invasion of Iwo Jima. Code name "DETACHMENT" was assigned to this operation. Marine staffs and commanders began to develop their plans and gather the forces for the invasion.
The Vth Marine Amphibious Corps was assigned as the major ground command for Iwo. Under MajGen Harry Schmidt, USMC, the VAC staff was an experienced body capable of complex staff work. VAC directed its subordinate units to be ready in all respects for combat by 15 December 1944.
The 3rd, 4th and 5th Marine divisions were assigned to VAC as the ground combat elements. And a wide variety of support units were marshalled to back up the divisions. Iwo was to be the largest Marine amphibious assault in history.