American Military History, Volume 2 by Richard W. Stewart

Slow base development at Leyte had forced MacArthur to delay the Luzon invasion from December to January. Nimitz in turn had to post- pone his target dates for the Iwo Jima and Okinawa operations, primar- ily because the bulk of the naval resources in the Pacific-fast carrier task forces, escort carrier groups, assault shipping, naval gunfire support vessels, and amphibious assault craft-had to shift between the two the- aters for major operations. The alteration of schedules again illustrated the interdependence of the Southwest and Central Pacific Areas.

The Iwo Jima assault finally took place on February 19, 1945, with the 4th and 5th Marine Divisions supported by minor Army elements making the landings. The 3d Marine Division reinforced the assault, and an Army regiment ultimately took over as island garrison. The marines had to overcome fanatic resistance from firmly entrenched Japanese who held what was probably the strongest defensive system the American forces encountered during the Pacific war, and it took a month of bloody fighting to secure the island. In early March a few crippled B–29s made emergency landings on Iwo; by the end of the month an airfield was fully operational for fighter planes. Later, engineers constructed a heavy bomber field and another fighter base on the island.