On D-Day, 15 June, Admiral Spruance heard that battleships, carriers, cruisers, and destroyers were headed south to relieve threatened garrisons in the Marianas. Since amphibious operations at Saipan had to be protected, Spruance could not withdraw too far. Consequently, a fast carrier force was sent to meet this threat while another force attacked Japanese air bases on Iwo Jima and Chichi Jima, in the Bonin and Volcano Islands, bases for potential enemy air attacks.